bitflipper
Any track can arbitrarily switch between mono and stereo regardless of the specified interleave. In fact, a single track can switch from one to the other multiple times as it navigates through the fx chain.
SONAR is just adapting to each plugin's requirements, some of which can only handle mono in and mono out, while others can only do stereo ins and outs. Some are hardwired to output stereo regardless of the input.
So yes, SONAR does see your mono track and starts out with a mono interleave. But depending on what you put into the fx bin SONAR might have to treat it as a stereo track, and you won't even know it unless it happens to be the last plugin in the bin. (And of course all tracks ultimately become stereo when fed to a stereo bus.)
Waves plugins normally avoid the issue by supplying separate mono and stereo versions of each. FabFilter does that, too. But not so many others. Maybe it's not the Waves plugin that's at fault, but perhaps something else that follows?
Been through all that. I specifically use the Bass Rider MONO
only in that fx bin. I also have the plugin properties to perform in mono mode.
SONAR did not detect the plugin and wave status to process this as a mono file. The onine help even states this:
When using a mono VST plug-in on a stereo track (interleave set to Stereo), the left and right channels will be out of sync. The left channel is processed by the mono effect, and delay compensation is applied, while the right channel is not processed and does not have delay compensation applied. The signal will look something like this:

Left channel: Wet signal (delayed)

Right channel: Dry signal (no delay)
A mono VST plug-in will work correctly if Enable Mono Processing is checked in the VST Plug-in Properties dialog box and the track interleave is set to mono.
This simply does not work as it would be best for a workflow. The above statement says SONAR is not intelligent enopugh to determine the track status. I am going to call TS and get some clarification and submit a request for enhancement.