When inserting a New Audio Track this button is set to Stereo for a Mono Audio Track.
I wonder why this is not set to Mono.
Feeding Track FX is also a consideration, do you feed it Mono or Stereo, this could be the reason.
When I toggle this button to Mono my Track Output Loudness increases (Mono Track, no FX).
Makes me wonder if sound data is going down the drain using up data cycles.