I do not believe this, my guess is that though AP is invalid after the GC approval but it should certainly remain valid before you surrender it, that is, when you get your PP stampped.
Otherwise, guys out of country with their GC approved will not be able to get in at all.
Logically thinking how could a legal resident be denied of entry at POE. Even though if someone's passport is not stampped, which the immigration officer will find out the moment he runs query against the file number at POE, if the person seeking admission have a re-entry document whichi is not expired by date, I don't see any reason why would he refuse entry. My guess would be that he might congratulate the person for the approval.
Find out details from your lawyer or some immigration expert. My feeling is that it should not be a problem.
Please use this advise on your own risk, I'm not a immigration expert or attorney, stated above are just my thoughts.
Thanks