Why are screen timeout settings being ignored? For the last month, 2 phones won't automatically sleep.

This is a new problem. Perhaps Google Play services did an auto-update last month?

Moto G5 Plus (Android 8) - music server

Moto G7 Plus (Android 10) - main phone

I have an identical "backup" Moto G7 Plus (Android 10) that remains turned off. I powered it on yesterday and display sleep worked fine (as it did with the above phones since forever).

I've googled for a day and have found no solution.