Thank you mikael and JonB for the help.
You sure you have update_interval set?
Yes. I have:
self.update_interval = 1.0
in my derived class's __init__ method.
It likely I did something to break code that was working, but I don't see anything wrong. I'll debug my application - if this works for others, then I'm sure I introduced a bug somewhere. It's odd though, because I only made two very small changes and then updated the OS.
Next time, before I upgrade the OS, I'll test before to make sure it's the OS that breaks code, and not my code changes.