The backup bug is still here, since 16.0 betas. if you backup locally to Mac or Windows, it will prompt you for a passcode about 60s after you plug it in or place it on a charging pad. This may be what you’re seeing, as I get it every day since 16.0 betas.
I call it a bug, because in 15.x and before, it would ask you once, the first time you plugged in to Trust This Computer. Now, it’s on a 50 first dates mode, where it can’t remember that you said ok and entered your passcode, it just asks over and over, every time.
The fact that it waits for sync to start and then backup to go before it asks for the passcode, means people are likely missing this, which will lead to backups being missed and eventually data loss.