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I tried looking at vote counts for an answer (with a net negative vote count) and for a question with positive votes. Both from the same thread.
For the answer I see this:
And for the question it works fine:
Is this a bug?
The answer has probably been deleted since you loaded the Q&A pair. As you don't have the "moderator-tools" privilege, that allows you to see deleted posts, fetching the vote count failed.
This is standard and known behavior, so status-bydesign.