Ok sorry for the scare - it looks like the actual errant behavior is related to one plugin (Recabinet 3.5.0) and it is X3b that actually is handling the plugin in a way that ends up boosting the track level by 3db.
Starting in X3b with my recorded (mono) track and no plugins, I add Recabinet. So far the levels are correct. When I save and exit the project and come back in, the track is louder by 3db. If I remove the plugin and then add it again the track is back to the correct level. Again if I save/exit and come back in, the track is louder. This does not happen in X3c. in X3c the track stays at the correct level thru saves (yay!).
One thing I noticed is that there is a visual indicator in Recabinet for panning within the plug - indicating the plug has been loaded in stereo mode. In the X3b save/exit reload behavior, this indicator goes away, indicating that the plug has been loaded in mono mode. So that's different. In X3c, the panning indicator persists through saves/exits and reloads. The X3c behavior is much more desirable, obviously.
Another benefit I noticed is that in X3b after doing the save/exit/reload, switching the track to stereo doesn't quite work as the plugin only seems to be affecting the left side (likely because the mono version of the plug is being used?) While in X3c I can toggle back and forth correctly between mono and stereo.
So in summary, it seems that X3c fixes some oddball behavior with mono plugins, or at least for the Recabinet plugin.
So I'm sticking with X3c and just adjusting my levels going forward.
Thank you all for your help.
Paul