It's been a while since I've used this plugin, (I've been largely away from the 'game' for a bit) and I don't remember this being the functionality. I'm stuck bouncing back to mono after the T/PS2 has done its thing. Anybody?
 
I'm using X3d Producer (I own the new version but haven't installed yet, want to finish this project before doing the whole Win 10/Sonar Plat upgrade thing).

Indeed I doesn't make sense. When you apply a mono effect to a mono clip, you don't expect to end up with a stereo clip. Just remember to set the track interleave to mono before applying the process. This ensures that the resulting processed clip will be mono too. I usually forget though, and it's annoying.

But I'm guessing that Sonar is not easily able to automatically predict in advance whether stereo or mono is required when applying a process to a clip. After all, sometimes you apply a stereo effect to a mono clip and want the end result to be stereo.