(JS) Strange behavior with forEach/shift

Relevant links:

JSFiddle playing with "secret santa":
https://jsfiddle.net/B1KMusic/gu4ofjre/3/embedded/result%2Cjs/

Modified version to scale it up, expanding on the "small number of chains" peculiarity:
https://pastebin.com/VUfbVGg5

While I was playing with the code to generate a huge string of unique names, I decided to see what happens if I replace arr[l-1] (the last element of the current generation, nondestructive) with arr.shift() (the first element, destructive)

arr = "abcdefghijklmnopqrstuvwxyz".split("");

for(let i = 0; i < 6; i++)
    arr.forEach(function(el){
        arr.push(el + arr.shift());
    });

arr.join(",");

The results were quite surprising:

'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,ymmgmgqmgqqimgqqiqiiemgqqiqiieqiieieecmgqqiqiieqiieieecqiieieecieececcb,ukswyymymmgymmgmgqymmgmgqmgqqiymmgmgqmgqqimgqqiqiieymmgmgqmgqqimgqqiqiiemgqqiqiieqiieieec,ieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyymecouukuksukswukswyukswyymukswyymymmgecouukuksukswukswyukswyymukswyymymmgukswyymymmgymmgmggd,ecouukuksukswukswyukswyymukswyymymmgukswyymymmgymmgmgqukswyymymmgymmgmgqymmgmgqmgqqiukswyymymmgymmgmgqymmgmgqmgqqiymmgmgqmgqqimgqqiqiie,mgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecouqiieieecieececoieececoecouieececoecouecouukqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukukkf,qiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyymieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyymecouukuksukswukswyukswyymukswyymymmg,ymmgmgqmgqqimgqqiqiiemgqqiqiieqiieieecmgqqiqiieqiieieecqiieieecieececomgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecooh,ieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyymecouukuksukswukswyukswyymukswyymymmgecouukuksukswukswyukswyymukswyymymmgukswyymymmgymmgmgqecouukuksukswukswyukswyymukswyymymmgukswyymymmgymmgmgqukswyymymmgymmgmgqymmgmgqmgqqi,mgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecouqiieieecieececoieececoecouieececoecouecouukqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukssj,ymmgmgqmgqqimgqqiqiiemgqqiqiieqiieieecmgqqiqiieqiieieecqiieieecieececomgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecoumgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecouqiieieecieececoieececoecouieececoecouecouuk,mgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecouqiieieecieececoieececoecouieececoecouecouukqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswieececoecouecouukecouukuksecouukuksukswecouukuksukswukswwl,mgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecouqiieieecieececoieececoecouieececoecouecouukqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyym,aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaan,ukswyymymmgymmgmgqymmgmgqmgqqiymmgmgqmgqqimgqqiqiieymmgmgqmgqqimgqqiqiiemgqqiqiieqiieieecymmgmgqmgqqimgqqiqiiemgqqiqiieqiieieecmgqqiqiieqiieieecqiieieecieececo,ecouukuksukswukswyukswyymukswyymymmgukswyymymmgymmgmgqukswyymymmgymmgmgqymmgmgqmgqqiukswyymymmgymmgmgqymmgmgqmgqqiymmgmgqmgqqimgqqiqiieukswyymymmgymmgmgqymmgmgqmgqqiymmgmgqmgqqimgqqiqiieymmgmgqmgqqimgqqiqiiemgqqiqiieqiieiep,qiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyymieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyymecouukuksukswukswyukswyymukswyymymmgieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyymecouukuksukswukswyukswyymukswyymymmgecouukuksukswukswyukswyymukswyymymmgukswyymymmgymmgmgq,ieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyymecouukuksukswukswyukswyymukswyymymmgecouukuksukswukswyukswyymukswyymymmgukswyymymmgymmgmgqecouukuksukswukswyukswyymukswyymymmgukswyymymmgymmgmgqukswyymymmgymmgmgqymmgmgqmgqqiecouukuksukswukswyukswyymukswyymymmgukswyymymmgymmgmgqukswyymymmgymmgmgqymmgmgqmgqqiukswyymymmgymmgmgqymmgmgqmgqqiymmgmgqmgqqimgqqiqir,ymmgmgqmgqqimgqqiqiiemgqqiqiieqiieieecmgqqiqiieqiieieecqiieieecieececomgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecoumgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecouqiieieecieececoieececoecouieececoecouecouukmgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecouqiieieecieececoieececoecouieececoecouecouukqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuks,mgqqiqiieqiieieecqiieieecieececoqiieieecieececoiee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ecoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswmgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecouqiieieecieececoieececoecouieececoecouecouukqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswieececoecouecouukecouukuksecouukuksukswecouukuksukswukswymgqqiqiieqiieieecqiieieecieececoqiieieecieececoieececoecouqiieieecieececoieececoecouieececoecouecouukqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyqiieieecieececoieececoecouieececoecouecouukieececoecouecouukecouukuksieececoecouecouukecouukuksecouukuksukswieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyieececoecouecouukecouukuksecouukuksukswecouukuksukswukswyecouukuksukswukswyukswyz'

How does this happen? keep in mind that we start with [a,b,c,...x,y,z] as iteration 0, and the following iterations are, at least theoretically:

[b,c,d,...x,y,z,aa]
[c,d,e,...x,y,z,aa,bb]
[d,e,f,...x,y,z,aa,bb,cc]

and so on until you have generation #2:

[aa,bb,cc,...xx,yy,zz]

and this goes on for only 6 generations to end up with the monstrosity above.

Have I discovered something vaguely reminiscent of undefined behavior (like how using memcpy or memmove in C results in UB if the regions overlap)? Does forEach have some strange nuance when it comes to modifying the list *while* iterating through it? Or is there something else going on?

Note: I am aware that generation #2 actually looks like this: [aa,cb,ec,gd,ie,kf,mg,oh,qi,sj,uk,wl,ym,aan,eco,iep,mgq,qir,uks,ymt,ecou,mgqv,uksw,ecoux,ukswy,ukswyz]. This question is about expected behavior vs actual behavior and how/why the results come about.