I don't know why this happened, but I can add some info.
It looks bounty and roomba related.
They placed a 500-rep bounty on their question on Dec 30, 2021 at 18:30. You can see the amount "paid" on that date on their rep graph. The bounty ended on Jan 6 2022. It looks like it wasn't awarded to anyone. What I don't understand is why the bounty would be refunded.
The question was roomba'ed as RemoveAbandonedClosed on Aug 31 2025 at 3:00:00Z- the same day, the user got 460 rep back for that question. There was a rep-recalc event on the user on 2025-08-31 at 9:55:00Z.
What I don't understand is how this lines up with the Help Center page on bounties (and in the page on the privilege to set bounties), which says
All bounties are paid for up front and non-refundable under any circumstances
And the post doesn't seem to meet the conditions for barring reversal of rep events related to the post.
Upon reading https://meta.stackexchange.com/a/16067/997587, it seems that refund of bounties on deleted questions is possible:
What happens if a question where I awarded a bounty gets deleted or migrated afterwards?
If a question on which you have started or awarded a bounty on later gets deleted, then in most cases, your reputation is refunded. It can take a few minutes for this to happen. If the question is later undeleted or migration is rejected, the reputation is deducted again.