nice findings mate. the bad news is that its surely hardware and its a combo of softkeys and display.
the reason that old roms are booting, is the kernel. older kernel has got other firmwares. Maybe somewhere when samsung was updating jf kernel, they killed your variant. i cant debug it so easy...specially now that i dont have your device variant
Do you have the GT-I9505?
This happens couple times even with 18 rom verGood afternoon, this is what happens to me sometimes when I take photos and then I see them in the gallery, the screen stays pink, then scratches, then the black screen, and then this message appears. System UI does not respond
expectView attachment 5622711View attachment 5622713
I've seen this in the past, too (but not in the last time). As @side already said, it would be great, if you could provide a log when this happens (either with a dedicated app - you have to be rooted for that or with adb - no root neccessary).
Open GApps are not officially support A12L yet. But there is Unofficial builds by Recognized Contributor. You can find it in my S4 Unified Collection & Guides or directly here.