Perhaps that is one reason why novels don't usually use first person with multiple different characters. Although it is done sometimes (I noticed my daughter was reading a Rick Riordan book that had two first person narrators), but it's possible that in those cases there is more than one storyform in play.
That's something to consider too, that many novels will have multiple storyforms, with the potential for different MCs. So with "tight 3rd person" perspective, which can be almost as close as first person, some or all of your POV characters might each be an MC in a storyform! (Note that each storyform doesn't have to be a full-resolution one; for example it could stop at the Domain/Concern level like the sub-story in Finding Nemo.)
On the other hand, tight 3rd POV characters need not be an MC, they may just be objective (OS) characters whose inner motivations you are describing. It would good to come up with a list of questions we could ask to determine whether a POV character is intended to be an OS character or sub-story MC. (Or possibly IC, though that really confuses me.)
I remember a background character in one series I read (Jim Butcher's excellent Codex Alera) who was a friend to the MC & "good-guys", but always in the background. Then there was a chapter from her perspective that suddenly explained how she was actually a spy, working for the bad-guys because they were keeping her daughter hostage. You could see this whole awesome character arc ready to play out, would she risk her daughter to do the right thing?, would the good-guys help her? etc. ... And then I was totally let down because that character was put by the wayside, at the end of the book her resolution was just narrated away in a couple sentences. Plot-wise all the questions were answered about her daughter etc., but nothing was shown from her perspective anymore. I didn't know Dramatica at the time but I now think it was an MC throughline that didn't complete properly.