I don't have any links to back it up or anything, but my impression is that Face ID continually trains itself as you use it to help keep up with facial changes

I don't have any links to back it up or anything, but my impression is that Face ID continually trains itself as you use it to help keep up with facial changes over time. At the very least I know it takes the data from failed attempts immediately followed by passcode unlock and uses it for training. Depending on how either is implemented I could see it getting "used" to a masked face with enough samples. The question for me is if this makes it more susceptible to false positives, e.g. unlocking for people other than the owner wearing a mask.