I downvoted this answer because the original revision was poorly formatted.
The author then edited it, to fix the formatting.
I still cannot cancel my downvote.
Note: I am not trying to change my downvote to an upvote, just cancel the downvote. This has always worked for me in the past.
Could it be to do with the fact that the author edited it less than 5 minutes after originally posting it, so the edit does not appear in the history?
Update: I was able to work around this by editing the answer myself.