If I actively try to put it to sleep with the power button, and not just wait for it to time out, it most often boots...
I had a suspicion to Greenify, that acts in this scenario, but stopping it before, does not change the behaviour.
ADD: Tried to disable the Accessibility of any apps granted that (3C battery monitor, Greenify, Android Assistant) - no difference. Any logs that could benefit anyone before I wipe it clean?
Went one step further and stopped every user app running, and found the culprit AutoNotification, that came in with my backup from Stock, after uninstall, and re