I placed RMix on a vocal track to remove some microphone noise and it worked beautifully! However, it is causing most of the midi notes of separate tracks not to be played back. For example, on a Sesssion Drummer 64 track the very first midi note plays, but then it is mostly silence after that. Occasionally another note will play but most are not played. The same thing is true with a Dimension Pro 64 instance being used for a bass track. If I simply turn off RMix in ProChannel then everything plays as it should.

Not sure if this has been reported or where to report bugs so I am starting here.

What is your midi buffer size set to? RMIX has a large delay so make sure that the MIDI buffer size is set to 512 (which is the default in X2). The setting is in preferences | playback and recording | Prepare using (250) msec buffers.

That did the trick, thank you very much.