I've had the same issue pop up here and there over many many years, sometimes even after exhausting any possible grouped items with compound shapes and masked items etc, it's just a shut down and restart job to 'fix' it.
I think it is ligitimately a 'thing' sometimes, that there is an obkect somewhere within the selection, but often just a bug. when it is a 'thing' it usually turns up due to what seems like another bug, where I have used 'paste in front' or behind, and the pasted item, doesnt actually appear in the same place as intended but waaaay off screen somewhere, yet still adheres to being pasted within a masked selection of items usually within a masked image area.
Posting to hear if there are other experiences and fixes..