Ok so here's the full story.

- DLC dropped, as expected it was Online Only. I usually take pictures of all the new vehicles for the infoboxes on GTA Wiki. Because the ScriptHookV is not working after updates and I prefer infoboxes with a car's default traffic color, I am the one who takes infobox pictures for DLC vehicles.

- I mod popgroups.ymt so that the game will spawn only the new lowriders in traffic (forgetting about extras like taxis, buses or trucks).

- I load the game and go to Director Mode for a temporary screenshot setup. I see a Minivan Custom so I run to it. It disappeared as I got closer. I thought it was some memory issue which removed the cars to free up resources (despite the entire LS being a ghost town).

- I find another Minivan Custom, I got very close to it. Even opened the door once. It disappeared anyway. So I steal a Taxi (that, weirdly, did not disappear) and drive further. I come across vehicles like Tornado Custom, Faction Custom Donk, and Slamvan Custom. They were disappearing too.

- I stopped in a middle of an intersection by the Mile High Club construction site. I shot down one driver of a Minivan Custom, quickly got into it and drove forward a bit. The Minivan disappeared while I was driving it, so Franklin was now standing on the street with no lowrider nearby.

- I make the post about the issue here. It didn't get much attention because it was the Online section.

- I decide to investigate further. I restart the game. Vehicles still disappear in BOTH Director Mode and Story Mode. I timed how often they do which was a consistent 5-6 seconds. In the scripts I found 3 new ones that I thought had something to do with it; mp_prop_global_block.ysc and mp_prop_special_global_block.ysc and mp_save_game_global_block.ysc. I made this post. I didn't know whether it was a really, really weird glitch or intentional so I added a disclaimer at the end, just in case.

- Frito-Man confirmed that the same thing happens on PS4 when you do a MP to SP glitch. So that's two people that can confirm this.

- I load up an old save from the very day when Executives and Other Criminals were released. I had a Cognoscenti (Armored) parked on the street, and all 4 Schafters parked in Franklin's garage. None of them disappeared. So I deleted the suspicious scripts but that only made the game load infinitely. I added this note into the same thread as my previous post.

- I take a look into ScriptHookV thread to see if others have the same issue, if not, let them know of it. Noone mentioned a thing about this so I made this post.

- Then I do a small experiment. I rename all lowrider models. Literally added an "a" in front of all the names, same with carvariations.meta and handling.meta, and then corrected vehicles.meta with the new names. Then changed popgroups.ymt with the new names. In game, the lowriders did not vanish. At this point I was fairly certain that this is intentional.

- In Script Hook V thread I was asked how I got mods to work if ScriptHookV is not working. ASI loader and OpenIV.asi do not require scripthook. I also further elaborated on the issue with the vanishing lowriders.

- _CP_ also came across this issue an hour before my first post.

- unknown modder then confirmed it too.

- Claude Liberty asked where are the scripts located, he checked it, turns out only one of the 3 I mentioned was new (even though I didn't see any of those 3 in the decompiled scripts list). He also found another one, scripttest4.ysc. Same results.

- People get mad at R* in a few threads.

- funmw2 shares the news to Twitter.

- The news end up in /r/GrandTheftAutoV_PC. _CP_ suggests to x-post it to /r/GrandTheftAutoV.

- unknown modder discovers the weapons aren't working. They actually are not blacklisted like the vehicles though. I could spawn and use them just fine the next day after ScriptHookV updated and I used the old trainer version. Guad and _CP_ could confirm this too.

- Alexander Blade updates ScriptHookV to new version. Now finding the cause can really begin.

- arewenotmen uses a function that disables scripts in-game. Turns out none of the suspected scripts have anything to do with it.

- _CP_ releases a mod that renames the lowriders.

- arewenotmen finds where the real car blocker script is located. It's in between magdemo.ysc and yoga.ysc.

- Rockstar must be dense as f*ck.

- nkjellman suggests a good way to get back at R*

- arewenotmen finds the cause. It's a "shop_controller.ysc" script. What it does is self-explanatory. It controls the shops in game. Disabling it f*cks up all the shops.

- sjaak327 almost quits Simple Trainer development as a result of Rockstar's dick move.

- sasuke78200 makes a script that disables that shop_controller script. Three-Socks later discovers the global variable that controls the blocker function and adds it to his trainer.

- funmw2 confirms that the discovered global variable is the one which controls the blocker function.

- _CP_ asks for a compiled asi with just the global var disabled. He eventually gets it and updates his mod, no longer requiring renaming. He requests lock on his GTAF thread for the mod.

- sjaak327 begins to update his trainer. It has the global variable disabled so the vehicles are now spawnable without modifying the game files.

- Crisis averted, for now.

 Might have missed something but that's pretty much it. Modders win again. Sucks that PS4/XB1 players will have to live with the car blocker though.