Man Im all out of ideas... sorry 😕

Just to be clear...   that track routed to anything other than the Guitar bus seems to pan OK?   Or does it also not pan correctly when routed to a new bus, and it's only the existing 2 Buss and Master where pan seems to work correctly?
 
What do the track and various bus meters look like under these combinations?
 
 

wanted to re-verify this so i checked again... 
I made a new bus... Bus I.  Mono track panned to stereo busses Master, 2 bus, bus I sounds normal.  When I say normal, panning full left in the mono track shows as a left signal only in the bus.  
Mono track panned to Guitar bus does not sound normal.  Panning full left on the mono track shows as a stereo signal (L & R) on that bus.  
It's almost like an fx is hidden in the bus, forcing the mono signal to be converted to stereo. 
Later, I tried more options to see if it may be some glitch.  
I clicked on the bus Post-fader and the track pan worked.  I turned it off and put the Kramer Guitar fx mono/stereo plug on the bus.  Track pan didn't work (acted like a stereo bus).  I turned off the fx, and it still acted like a stereo bus.  I removed the fx from the bus, with the same results.  I selected post-fader, and the bus acted normally.  I de-selected the pf button, and the bus remained normal.  
So it seems like the plugin locked the signal to the bus in stereo mode?  
It's getting late so I'll look at it more tomorrow.

That does sound like progress.    If it's narrowed down to that guitar bus, but visually there are no plugins in either the FX Bin or Pro Channel and no sends, it could be a project worth sending to the bakers.  Particularly if you can pare down the project to one audio track and a few busses.

If they want to see it, then I can do that... I noticed that this project used my default master template and I can replicate it on other projects using the same template.
 

Cool, well, sorta.    Any chance you could upload your template for more eyes on the issue?

I would occasionally get a wierd artifact like this when I would open an old session.  Try cloning the track with no details, just the event.  No effects, automation...etc... just the event.  Right click on track 8 - "clone Track" - choose only events.. not automation etc.  Im sure theres a reason... however I never dug deep to figure it out

I don't know what you'll see from this unless you have all of the same plugs as I do?  I don't know how that works, but here it is.  Appreciate the look.  I thought about filling out a problem report but I have to write out every step required to duplicate it... it'd prob take me over half an hour to do it.
 
https://dl.dropboxusercontent.com/u/23691067/Latest%20Master.cwt
 
let me know if your plugs load into the new project that you make from this template.
 

OK, this may seem a bit odd, but, if you toggle the Guitar bus to mono and then back to stereo, does that fix the issue?   It does seem to work here, but I also don't have a lot of plugins from your template so it's not completely a one-to-one comparison:
 
MaxxVolume Stereo (Bus: 'Master')
T-RackS CS (Bus: 'Master')
PAZ- Analyzer Stereo (Bus: 'Master')
T-RackS CS Metering (Bus: 'Master')
iZotope Nectar 2 (Track: 'Vocal Lead')
DeEsser Mono (Track: 'Vocal Lead')
IR-L efficient Stereo (Bus: 'Reverb')
REDD37-51 Mono (Bus: 'Parallel Drum Comp')
TG12345 Mono (Bus: 'Vocal')
RBass Mono (Bus: 'Bass')
Renaissance Axx Mono (Bus: 'Guitars')
EKramer GT Mono/Stereo (Bus: 'Guitars')