My place had an on-train mask altercation just this afternoon, so it is happening.
The risk of trouble should certainly not be dismissed lightly, you’re so right that tensions are running increasingly high, and the hot weather doesn’t help. I don’t think masks is necessarily the major issue, merely that people are fed up and worn down generally, with masks being simply one potential flashpoint. A decent PM could have done so much more to stop cohesion falling apart, instead he seems to almost enjoy watching it happen.