Studio One gives you the option to set each track to stereo or mono.
Typically, I work in mono for vocals, bass, and kick. However, mono makes them sound smaller and less staged.
Are you using all of the above settings in mono?

 

I occasionally do mono checks to make sure it’s mono-compatible, but will always prefer the final mix be in stereo, as much as I can, to me, that means mostly stereo tracks.

as you noticed, making those busses mono, it loses its side information, which can it feel more “narrow”.
You have to compensate for louder volume if you’re making certain busses completely mono to make it sound more balanced in your song. This way, your harmony and overall balance of your mix can still be intact.

depending on which genre, maybe it’s best to leave some of those busses in stereo to retain that side information.
It doesn’t invalidate your current workflow tho.