I have the same problem, and no, I don't think this has anything to do with setting the firmware password.
I have a MacBook Pro (Retina, 15-inch, Mid 2015). I just updated from Mojave to Big Sur. The update completed, then I stepped away from my Mac for several hours. When I returned, the screen was locked, as expected, but instead of asking for my password, it asked me for my "PIN", which I don't ever recall setting. I tried entering my usual password, but it failed. When I clicked on the "?" next to password field, it gave me the password hint associated with my password. It also said if I forgot my password, then click to restart the computer, which I did.
I'm confused. Why is it asking me for my PIN? And, if it is some PIN that I set somewhere, why is it giving me my password hint? The firmware password (which I had never heard about til now) screen is a black screen with a picture of a padlock. I have never seen that, and I don't see it now.
Help!