When I updated my world from 1.15.2 to 1.16.1, most of the decorative player heads I had around my house turned to the default Steve skin! Strange thing though, a few did not change. What's going on? Screenshots include the same room in 1.15.2 and 1.16.1.

However, I have a copy of the same world that I have been regularly using to test out snapshots, and when I played that copy in 1.16.1, the heads were unchanged. So, something about the big jump from 1.15.2 to 1.16.1 did it!