After setting a KLCK preset and saving it I've turned off my screen waited at least 30 seconds before waking the device but I'm not getting any KLCK or stock lockscreen, just whichever launcher homescreen I'm using (lawn chair V2 or Nova) but the screen darkens and freezes up until you press the power button again.
KLCK is turned off on battery optimization settings.
Noted happening on following devices.
Honor 7S with EMUI 8.0 and Android 8.1.
Alcatel 3V with Android 8.0
Nokia 3 with Android 8.0
Motorola Moto E5 with various non stock custom roms running Android 8.1 or 9.0
Samsung J4+ on Android 8.1
Samsung J6+ on Android 8.1
Working fine on Sony Xperia E5 with Android 6.0 Marshmallow and Sony Xperia L1 with Android 7.0 Nougat.