Huh, you're right - I just did what you described, and was able to change the default application for .html files. Weird! I could have sworn I did the same thing a few times, and it kept reverting back to Safari.

Apparently this was an issue with the user, not the OS. My bad.

EDIT: Actually, I remember now that I had to use duti to change this from the command line.

https://formulae.brew.sh/formula/duti