As I take a look on the keyboard driver (on the STM32) its lack of checks on start…
What I can say is if just one bit start high, you can have this kind of glitch, without being noticed, until you fully power reset.
Your behavior seem validate my analyze.
That’s how “stable” the STM32 is!
(I don’t want to put some ads, but since you use the keyboard frequently, you could try my custom firmware and see if you still experience this “slow” glitch or not. What I can be sure of is that there are no duplicates key press on it.)