Multi Channel to mono works for me as long as the stereo audio is not in musical mode.
If the source audio is in musical mode - even at the same tempo as the tempo track it does not work. It results in two tracks being created with an extremely short piece of audio on both.
If I use sizing applies time stretch on the audio - multi channel to mono does work.
I’m on El Capitan which isn’t officially supported so it may be because I’m not on Sierra / high Sierra