Apps slipping out of their preferred spaces

Since upgrading to BigSur and with TS 2.9.6 I’m having an issue with apps slipping from their preferred spaces and moving into different spaces on a different monitor. It only seems to happen when the displays have been asleep - and the prime culprit is Apple Music. It’s easy enough to drag the app back to where it belongs, but it would be better if it stayed where it’s supposed to!