Problem with "Repeat Keys" Setting

I’m having a problem where the “Repeat Keys” setting (under “Universal Access”) is sometimes not doing its thing - but then going into the settings, twiddling with either the “Delay” or “Speed” sliders will magically fix it again.

This is the setting that controls what happens when you hold a key, e.g., when you hold backspace to try to delete loads of stuff quickly.

On a fresh boot, everything will be fine. Then, out of the blue, it’ll be as if someone set the delay and speed to the slowest settings (holding backspace will take ages to delete anything).
Like I say, twiddling the settings a bit, even just a pixel or two, instantly remedies the problem.

It’s good that I have a workaround, but it’s a bit annoying. Is there a way to debug what might be causing it? I can’t reproduce it at will, and I can’t figure out what I’m doing that’s causing it. It feels random, but we know it’s probably not!