Is there an official ruling on that somewhere? Otherwise, I think it should be considered a new roach; why would Maggie be swatting roaches onto the countertop? I think the reason for respawning them is just so that you can't kill all the roaches and complete the encounter before succeeding in getting Maggie's attention.
That actually makes a lot of sense, particular since I found this http://www.plaidhatgames.com/sum_forums/showthread.php?2099-... where Jerry (the game designer) says that the roach is counted as a kill towards the achievement! So re-spawn it is then.