Closing in on the "ZPRINT MYSTERY". It seems the culprit is the deep sleep mode of 64 bit iDevices.
Whenever the device goes into deep sleep mode the value of the _debug_enabled/_debug_allowed flag is reset.
<stefan.esser@sektioneins.de>

Idle iPad for several hours
---------------------------
$ ./kernmemread -s 0xffffff801c658a60-0xffffff801c602000+kernelbase -l 12 -f arm64
0xffffff8013258a60:	adrp		x8, #0xffffff8013710000
0xffffff8013258a64:	add		x8, x8, #0x6d0
0xffffff8013258a68:	ldr		w8, [x8, #0xd8]

$ ./kernmemread -s 0xffffff8013710000+0x6d0+0xd8 -l 4 
0xffffff80137107a8: 00 00 00 00                                     ....

$ reboot
Connection to 127.0.0.1 closed by remote host.
Connection to 127.0.0.1 closed.

Now ipad is freshly rebooted
----------------------------
$ ./kernmemread -s 0xffffff801c658a60-0xffffff801c602000+kernelbase -l 12 -f arm64
0xffffff8014058a60:	adrp		x8, #0xffffff8014510000
0xffffff8014058a64:	add		x8, x8, #0x6d0
0xffffff8014058a68:	ldr		w8, [x8, #0xd8]

$ ./kernmemread -s 0xffffff8014510000+0x6d0+0xd8 -l 4
0xffffff80145107a8: 01 00 00 00                                     ....
                                     ....
$ ./sleeptight
$ Connection to 127.0.0.1 closed by remote host.
Connection to 127.0.0.1 closed.

Now we just wake it up from deep beautysleep and reconnect
----------------------------------------------------------
$ ./kernmemread -s 0xffffff8014510000+0x6d0+0xd8 -l 4
0xffffff80069107a8: 00 00 00 00                                     ....