This seems to have been a different problem than I'd originally thought, so it's been posted as a new bug. The remainder of this answer can probably be ignored, but is left here for reference.
The adjustement described in Nick Craver's answer does not seem to be working displaying correctly:

This should sum to 215, but it sums only to 205, or 190 without the accept points.
That said, it's hard to debug what's going on here and be totally sure that the issue was that downvote. It look as if +10 was latter added for the upvote just after the downvote but 10 was somehow removed from the scores before the downvote; previously the scores had looked like this:

Note that in the first image above we have a total of 190 points for upvotes before the downvote, but in the second we have a total of 200 points before the downvote.
Is there any way to get the vote-by-vote timeline back again? I saw it once for part of the day, as you see in the second image, but I cannot get it back again. (Had I known this information would vanish forever I would have captured part of the titles in the screenshot.)
The log for the day from /reputation
shows:
2 22183 (10)
2 22183 (10)
2 22204 (10)
2 22183 (10)
2 22204 (10)
2 14016 (10)
2 22210 (10)
2 22210 (10)
2 22183 (10)
2 22183 (10)
2 22197 (10)
2 22200 (10)
2 22210 (10)
2 22210 (10)
2 22210 (10)
2 22210 (10)
2 22204 (10)
2 22183 (10)
2 22210 (10)
2 22183 [10]
2 22210 [0]
2 22210 [0]
2 22207 [0]
2 22210 [0]
2 22197 [0]
2 22210 [0]
2 22200 [0]
1 22200 [15]
2 22183 [0]
2 22200 [0]
2 22204 [0]
2 14016 [0]
-- 2021-11-09 rep +215 = 18790
I'm not entirely sure how to interpret the above data (the meaning of parentheses vs. square brackets is particularly mysterious), but this properly shows 215, my reputation total as displayed in the header is currently 18,790, and 18790 - 18575 = 215. So perhaps the problem on my user page is a display issue?