Unfortunately that doesn’t appear to be the cause either. I think there’s still something else adding to the mix as well that’s causing it to occur in some but not all cases.
To test a theory out, is there one you could reopen and then close again to see if that shakes the votes loose? (it should also give them back if they’re moved category to a non-voting one as well, which may be worth a test. And then move back to see if the problem persists)