I have no issues with SuperTap or any other Waves Stereo plug-in used in the FX bin on a mono track in SONAR with stereo interleave enabled.  They work exactly as intended.  I'm currently up to date with the latest releases of SONAR and Waves but have been using these plug-ins for a year or two with no issues.  
 
I just tested by creating a new project, enabling an input with a guitar plugged in, adding SuperTap 6 Stereo and panning two delays hard left and right.  All worked fine with no tricks.  Do you have any other plug-ins on the track that could be complicating things?  Have you tried testing with a fresh empty project?
 
This won't work in the Clip FX bin but I don't think any stereo effects work on mono tracks in the Clip FX bin because these are processed prior to interleave according to the signal flow documentation.
 
Again, do not bother with the Mono-To-Stereo plug-ins as these do not work correctly in SONAR and, as far as I know, never have.  I could've sworn that I've seen official confirmation of this at some point (from Waves if I'm remembering correctly) but I can't find it now.  
 
Like I mentioned before, sometimes SONAR and Waves get out of sync when you add a Stereo plug-in and then switch the interleave.  That's all I can imagine could be the problem.  Toggling the effect off and on again usually solves this though maybe you could try deleting it and re-adding it completely after ensuring stereo interleave is on?
 
I am using the VST3 versions if that matters.

Dave76
 
I just tested by creating a new project, enabling an input with a guitar plugged in, adding SuperTap 6 Stereo and panning two delays hard left and right.  All worked fine with no tricks.  Do you have any other plug-ins on the track that could be complicating things?  Have you tried testing with a fresh empty project?

 
I tested on a new project and SuperTap 6 Stereo worked perfectly. 
 
I then went back to the existing project - no other effects on the track, sent direct to the master, interweave on. Tried switching plugin on and off, tried switching interleave to mono and back. It does not work - just mono. Completely stumped. 
 

Marshall
I tested on a new project and SuperTap 6 Stereo worked perfectly. 
 
I then went back to the existing project - no other effects on the track, sent direct to the master, interweave on. Tried switching plugin on and off, tried switching interleave to mono and back. It does not work - just mono. Completely stumped. 

Wow, interesting.  Is it just SuperTap that doesn't work on that project or do the bundled SONAR delays do the same thing?  Nothing potentially strange with that project that would reduce it down to mono?  Does "pan" on tracks work?  If you are able to reduce that project down to a minimal example and get it to Cakewalk, maybe they can fix it if it's a bug.  

Dave76

Marshall
I tested on a new project and SuperTap 6 Stereo worked perfectly. 
 
I then went back to the existing project - no other effects on the track, sent direct to the master, interweave on. Tried switching plugin on and off, tried switching interleave to mono and back. It does not work - just mono. Completely stumped. 

Wow, interesting.  Is it just SuperTap that doesn't work on that project or do the bundled SONAR delays do the same thing?  Nothing potentially strange with that project that would reduce it down to mono?  Does "pan" on tracks work?  If you are able to reduce that project down to a minimal example and get it to Cakewalk, maybe they can fix it if it's a bug.  

Ha! Sonitus Delay, Cakewalk Tempo Delay, BT Tempo delay all work correctly. Both Waves SuperTap and Waves H Delay are in glorious mono - a pattern is emerging, BUT both Waves plugins work correctly in this project as Sends (not as Inserts) and they behave correctly in the new blank project. 

Using VST2 or VST3? Does it make a difference?

did you ever receive a reply from Waves support ? They are usually pretty fast at replying.
I was going to myself, but you had mentioned you were going to.
 

chuckebaby
did you ever receive a reply from Waves support ? They are usually pretty fast at replying.
I was going to myself, but you had mentioned you were going to.
 

I haven't yet, but will do now based on the evidence gathered. Will report back. 

My message to Waves tech was:
 
I have raised this on The Cakewalk Sonar forum (topic is SuperTap Question). This was last updated a few minutes ago and the general consensus is that I should raise it with you. 

In a nutshell, I have a Sonar project where if I use SuperTap Stereo 6, or H Delay Stereo as an inset, the output is in mono. The track has Interleave set to stereo, it is a mono source, sent directly to the master bus with no other insert effects or sends. 

I can drag the same instance of either of the Waves plugins to a bus and create a send from the above mentioned track, and it works perfectly, i.e. in stereo rather than mono. (Taps 1 and 2 are panned hard Right and Left and it is obvious)

On the same track, the Sonitus Delay, Cakewalk Delay and BT Tempo all respond correctly in stereo as insert effects. 

The odd thing is that I have created a brand new 'test' project with nothing on it, and both Waves plugins performed correctly. However, there must be something wrong as the non-Waves delays are all working correctly on the original project whereas the Waves ones are not. 

Finally, I have the same issue with Doubler 4 Stereo. 
 
Their response:
 

I could not recreate this issue in our test machine. 
 
It seems that what you are describing is a result of a corrupted session. 
 
You have already mentioned that on a new session this does not happen and the stereo routing behaves normally. 
 
You could try to verify that this session is corrupted:
Open other old sessions of Sonar and see if this issue remains or not.
Although, if those sessions were originally created on an older version of Sonar, you will have to verify there is complete version compatibility between them. 
 
Also, On the same session, please try to open another Stereo Channel (from scratch) , add only a Stereo Waves plug-in and see if the issue persist. 
 
Subsequently, I have created another track on the original project, added a Stereo Waves plugin and it works correctly!