There is a bug somewhere in there. I've noticed the same. It will hopefully be fixed when we update to the next release (releases are rapidly occurring at the moment).

In the mean time if you toggle your vote on your own post off and then on again, you'll see the correct number

Lemmy is very buggy with stuff like this. Further reason we need more programmer socks wearers here!

This may or may not be a related problem, but Lemmy does not show accurate sub counts for communities outside of this particular instance (i.e. ones made on lemmy.ml, lemmy.world, etc.). I checked beehaw.org, and the same problem occurs there. However, upvote counts and comments seem to be the same across instances (when not taking defederization into account).