I have the galaxy s9 plus and after one of the last few updates my screen timeout now bugs. I set it to 10 minutes but then it reverts back to 30 seconds. Sometimes within an hour or so other times overnight. Please help this is very frustrating for a simple setting.
As a process of elimination can you put the phone in Safe Mode which disables your downloaded apps and leaves the stock ones working and then see what happens.
If the issue stops it could be one or more of your downloaded apps. Come out of Safe Mode and start deleting your most recent apps one at a time until it stops.
If it continues in Safe Mode then what I'd suggest to do in order is....
Turn the phone Off and then from a set series of button presses go into the phones system files cache and clear those. No info is lost.
Back up , remove any Sd card and factory reset.
then a Samsung Experience Store or Samsung Service Centre can help.
Current Phone ~
Samsung Note 20 Ultra 5G SM-9820 _ Samsung One Ui 2.5/ Android 10. Snapdragon 865+