Weird stereo phase issue on master bus when inserting plugins

Hello there fellow Sonar users.  I'm new to the forum but have been a Sonar user for many years, started with cakewalk when it was just a midi sequencer and now am using Sonar Platinum on Windows 10, my audio interface is a Universal Audio Apollo, ASIO drivers.
Everything had been working fine but recently I bought a few plugins and ran the VST scan in order to use them.  Now, when I insert a plugin in the master bus, the middle seems to disappear and the signal from the right side gets a few dB louder and with a 20-40ms delay, so it sounds like a slapback delay applied to the R.  When I disable the plugin, things go back to normal, but I can't understand what is going on.  Like if I was using a mono plugin in a stereo track or something weird.  However, this doesn't happen with all my plugins, maybe half of the ones I tried (Chandler Curve Bender, Blue Stereo Chorus, Lexicon 224, Pultec Legacy and a few others, some that are clearly stereo and some that used to work regardless of applied to a mono or stereo bus or track).
 
What have I overlooked?!
 
Thank you in advace for your time and replies

The plugins I bought included the Chandler Curve Bender and that was the plugin I wanted to put in the master bus (I added other plugins then just to see if it was the curve bender causing a weird kind of a M/S effect or not, not that I would have a chorus or reverb permanently on the 2bus)  I also have an external send permanently on the master bus that goes to an SSL stereo bus compressor but that is working fine, no issues with the external send, just with a bunch of plugins.  I noticed it not only happens on the master bus, but on any other bus as well but didn't do too many tests as I had a recording session about to start when noticed this.  If I disable the plugins things go back to normal but when I enable a plugin, the metronome disappears from the middle and goes to the sides with a slight increase in volume and a short delay on the R side.  This had never happened before and I used to apply the UAD Pultec Legacy sometimes in the master bus or the UAD fairchild sometimes but now when I add them I get this issue.  Very strange and I can't seem to put my finger on what is happening.  

I tried that, but it didn't solve the issue.  I thought it could be that something happened when running the VST scan, maybe it failed to scan an old plugin or something so I started with a new blank project.  I inserted only 1 plugin in the master bus, armed a new mono track and started recording.  Metronome comes out of the L+R, louder on the R by about 2 or 3 dB and with a very short delay on the R side.  I disable the plugin and the metronome goes back to the middle.  Tried this with a bunch of plugins, some I would normally not use in the master bus, but just to see if it was just the UA Chandler Curve Bender, which was not cheap and I really wanted to use on my master bus.  Some plugins gave me the issue, some didn't, I tried about a dozen and I would say that more than half had this issue.

Yes, a new blank project, master NOT routed through the SSL.  I checked the routing in the audio interface and don't see anything strange.  I restarted the computer, didn't solve the issue.  Rescanned VST's, didn't solve the issue.

when scanning the vst, make sure to do a reset in the scan options. just to be sure. also, presumably this is simply a stereo bus, maybe what's happening is the latency reported by the plugin (and perhaps via the UAD hardware?) is off or offset. not a UAD user but perhaps there is a setting to adjust this for UAD? maybe turning off PDC to test? http://forum.cakewalk.com/PDC-override-full-explanation-m2575563.aspx
 

This isn't likely to be a PDC issue (e.g. a plugin mis-reporting its latency to SONAR) because the problem goes away when you bypass the plugin. Usually, incorrect delay compensation persists until you actually delete the offending plugin. Also, AFAIK PDC is per-track and is not applied to left and right channels separately because plugins don't report left and right latencies separately. 
 
This all suggests the problem is within the plugin, or conceivably another plugin that's in series with it on the same bus. Something is causing a delay between the left and right channels, and I don't think SONAR is even capable of that. 
 
You should be able to definitively eliminate (or finger) PDC as the culprit by creating a new project with one track, one bus and one plugin on the bus. The final output should sound no different whether PDC is enabled or disabled. 
 
This is a long shot, but have you verified that the bus interleave is set to stereo?
 
Can you duplicate the problem using only a single plugin? It could be a completely different plugin that's actually causing the problem, rather than the one you remove to make the symptom go away. 
 
If the plugin you're testing is VST3, is there a VST2 version of the plugin you could substitute?