Mono mic set up, stereo fx, weird sympathetic resonance stereo

Just playing around - I wanted to see if I could have one of the electric pianos in mono (either a mono mic set up, or mono output) and then to insert a stereo fx such as chorus or delay. I'm just thinking of those guitar fx pedals which have a mono input, but a stereo output. The reason was that I wanted to keep the purity of a mono output from the electric piano itself.

Well, I noticed the following:

1. Mono output  - stereo fx stay mono.

2. Single mic with both left and right outputs configured the same - stereo fx stay mono

3. Two mics mirrored exactly (one on top of the other and same mic models) left and right db the same - stereo fx stay mono

4. Two mics mirrored exactly (one on top of the other and same mic models), but, with one channel 0db and opposite for other mic (so, mic 1 outputs left channel only and mic 2 outputs right channel only)- stereo fx work in stereo. This maintains the mono output, but stereo fx come into play, even reverb is stereo.This was what I wanted to achieve, except see point 5.

5. Interesting and curious finding:  In setting 4 above, sympathetic resonance (S.R.) outputs in stereo. Now this is weird. Why isn't the S.R. mono.

Upshot is that if I want a mono signal plus stereo fx, there will always be a weird stereo S.R. pre the stereo fx. Is there a way around this perhaps?

I'll upload a preset for people to look at called "Weird Sympathetic Resonance." I've used a grand piano to exaggerate the S.R.

Happy to hear your thoughts.