When you look at playback info it should have basic info why it is transcoding. During playback bring up the controller bar and select settings icon ( like an equalizer) then scroll down to Stream info then to the right focus on Playback . While that is highlighted, at the bottom it should say why it is transcoding.
I think you are right this may be the reason but probably need to more detail about one of the files. Like the level. You can look at its XML from the web app https://support.plex.tv/articles/201998867-investigate-media-information-and-formats/#toc-1
