I have never seen this behaviour before

I've noticed the same thing intermittently, but haven't struck it for a few days now.
On one occasion when it came back with the other ++ radio buttons checked, I didn't actually have any votes left to cast - but I hit the "Vote" button again anyway, just to see what would happen, and I'm sure they registered !! (Didn't get me any additional xp, but.)

The next day, when I got around to thinking about reporting the strange behaviour, I was no longer able to reproduce it ... so I didn't bother.

Cheers,
Rob