If I copy the art in one document and paste it into another document, I will get the "Save changes to the AI document before closing?" prompt when I attempt to close the document from which the art has been copied, even though I have made no changes.

AI never used to do this to my recollection. It's a nuisance and causes doubts when I've been distracted from one task, only to come back and have to verify whether any changes have actually been made.

The majority of the time, no changes have been made but to be on the safe side, I have to verify each time.