This has persisted across several o/s updates including the latest (8 Nov 2024, 2.5).
If I press the power button to shut the display down, then it stays off for a lot longer but after an unpredictable length of time it will come on again for no apparent reason.
Check Settings/ Display/ Lock Screen/ Wake screen for notifications; if that is on the screen will turn on for notifications.
Well, that has worked!
Note that I don’t use a lock-screen, so the issue presumably had nothing to do with that.
And it didn’t always happen, but started about six months ago.
Maybe there is a rogue app that I installed that is issuing very frequent and regular ‘silent’ notifications?