There is an obscure setting in the Prefs area that says 'Allow mono tracks to use DX FX in stereo' (or something close; going from memory).

You mentioned you are using FX so thought I'd mention this setting. Mainly because I have no idea what it does! I've never seen it make a difference in any of my FX that I use, but maybe it's an option to try.

Also, I would suggest removing FX (especially new unfamiliar ones) from the tracks one at a time and see if one of them is causing the issue. Many of us have found weird issues solved by removing a rogue plugin that just wouldn't play nice on our system.

Make your sound your own!
.. I do not work here, but the benefits are still awesome