I was diagnosed with ALS in August 2019, with symptoms appearing one year prior to that. There was a time sometime after the diagnosis when I noticed I was clenching my jaw and grinding my teeth at night - I suspect it was stress over the situation and the fact that moving in bed was getting more difficult for me. I don't notice that I'm doing that now. It could be that I now have a hospital bed which makes me more comfortable while sleeping so I don't have to move around as much.
I do now bite the inside of my cheek occasionally at night. I suspect my facial muscles are weaker and I sleep with my head tilted to one side, so one cheek likely collapses inward a bit. Then, anytime my teeth come together, the inside of the cheek can you get caught.