Moreover, it also prevents unnecessary windows from creating chaos or slowing down your productivity.
Looking at the issue, it seems to be due to a software bug.
Besides all these usual suspects, I wouldnt rule out the compatibility thing.
Is Your Mac Compatible?
To get going, verify your Mac is compatible for this feature.
In most cases, it’s possible for you to resolve it by just turning off/on the feature.
Hence, I would recommend you disable/enable Stage Manager on your Mac.
Update Software on Your Mac
Finally, try updating the software on your Mac.
If none of the tips has worked in resolving the issue, go for the software update.
As already stated above, there is a strong likelihood of a software bug.