Edit: Problem solved. The issue was the Finale was automatically assigning LoopMIDI as a MIDI input, while I was using it as an output to send MIDI data to Reaper. It caused a feedback error which ...
Reinstalling didn't help, nor deleting preferences, but deleting the entire Finale 27 folder in %appdata% along with another reinstall seems to have done the trick.
Thanks for your replies. I tried looking in Preferences, Simple Edit options, and Document options for any setting that might look relevant but it doesn't seem like it's something you can change, w...