Serum MPE X not working with Serum

Ok! On my new sensel morph with free bitwig 8 track, I was able to get MPE working with Sensel express presets. I figured out that mpe has to be enabled in serum, then I was able to do some fun mod matrix assignments. MPE Y works great, MPE pressure works great, but MPE X (side to side) doesn’t register visually on the mod matrix and seems to have no effect. I looked through the sensel express presets for serum and don’t see any MPE X assignments, but I do sometimes hear a side to side pitch change. I am using Buchla Thunder. I am confused. Any ideas where I went wrong?