I tried googling this to see if anyone has had a similar idea but couldn't find anything, so I apologize if this has been brought up before but:

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I believe that Dennis is a group hallucination that occurs from the sex doll in season 13 episode 1. Hear me out. Dennis magically appears after the gang has been expressing how deeply affected they were by a silent, inanimate sex doll - and are now physically manifesting this (Dee under extremely saturated white light, Mac working out because of "fat" "comments", etc.). Dennis shows up wearing the exact same outfit as the doll. I think it's possible that this season's Dennis is just the group's way of dealing with the stress of a Dennis-less gang. I think it's highly likely, since Dennis does not explain why, or for how long, he is returning. Dennis has a lot of reason to stick with his girl and child, so it's highly likely the real Dennis is still there with his family.

It's just a theory!