Lost access to Dr. M, can't get Monster Professor fragment?

Hey guys, this could be me being thoroughly silly and missing something entirely, but I'm having some trouble and I'm curious if anyone has any advice. So, I recently reset Academia 4xx AF, because I missed Vanille's Fruit in New Bodhum ???. That went well and I got the fragment, but now I'm having a problem back in Academia. After closing the gate and redoing the level, I'm no longer able to talk to Dr. M, the guy you need to talk to to get the Monster Professor achievement. I know he's there (he has some dialogue about monsters whenever I pass him), but I have no way of talking to him and get updates on the achievement. Is this a normal thing? Did I mess something up? Anybody have thoughts?