The older Sonar version levels have changed since rebooting the PC.  I uninstalled Sonar, using the CCC, rebooted the PC and then installed the current version.  There is still about 0.75 db difference in the channels.  Realistically not much but there should not be any difference - both channels should be exactly the same.  If I mixdown to a mono track and then convert to stereo both left and right levels are exactly the same.  This would point to the Bounce to Track(s) stereo conversion as the issue (I'm using the defaults in the Bounce function).
 
I tried this with Studio One 3.  I have a version I got with a Presonus hardware purchase but don't really use.  I exported each mono track from Sonar and imported those tracks into Studio One 3 and then did a mixdown and both stereo channels are the exact same level (what is expected).   
 
FWIW I can't say this is a new problem. It may have been there all along and I never noticed it before.