I was having the same issue, though from a completely different kind of session and setup. The point was that panning simply didn't work at all. Going hard right or left only raised or lowered volumes of the track. None of the issues Alex talked about here solved it as all my settings were correct.
What DID solve it was turning on mixer bypass in the Universal Control AI app. I had turned this off to use my Studio 192 with Pro Tools, and had also had to make changes in Apple's Audio Midi Setup app. Once I got back to my standard Studio One settings (Mixer bypass ON), all panning worked as normal.
As an aside, I will just say that every time I have use Pro Tools (I got a month to month subscription so I can convert some old projects to use in Studio One) something goes wrong with my system.