I don’t know anything about the ATEM or Pro Presenter, but perhaps it’s an issue with loopMIDI? I know that back in the day when I used it, it would do some weird things if I tried to have a single loopMIDI port that did both send and receive, so I would have to create 2 loopMIDI virtual ports, one for send and one for receive. Don’t know if that has anything to do with your issue, but something to maybe check.
I found that out when using loopMIDI with one of my other pieces of software, YammieQ.
https://discourse.checkcheckonetwo.com/t/yammieq-instructions-windows