Yes, mine sort of did something similar. There was a group of 5 and they did single one out of the group sometimes, not always. When I would find the dead ones, if I could that is, they usually looked pretty chopped up(not just a dead fish or normal decomposition). So, kind of just waiting till he finds the last one to have for dinner. I may try and put him in my 29 I'm starting soon, I have to partially break my 120 down to get some of the fish, rock and other things, we'll see. Good luck with your specimen, I hope he stays well behaved for you. On a side note, let me know if you notice any "clicking" at night time now. I have a few rouge critter in my tank, don't think it is a Mantis, but was just curious to see if they can "click" too.
Matt