I've experienced this almost daily on Stack Overflow, and now I believe I've recently figured out the pattern of this bug.
- over 10,000 reputation if the low quality post is a question (it almost never happens)
- over 20,000 reputation in general, in order to have access to delete votes for answers.
- not a ♦ user, because you need a finite amount of votes for this issue.
Refreshing the page demonstrates that the review got invalidated:
For reference, the review item was: https://stackoverflow.com/review/low-quality-posts/24697740. As I'm not a moderator, I do not know if the original flag became "helpful", "disputed" or something else.
It may be possible that it actually happens for the LAST+1 Delete vote (I didn't count precisely), but in any case it seems like some off-by-one error somewhere: if it's the last+1 vote, then the review queue should be considering it as "Recommend Deletion" instead of invalidating the review.
Please accept my apologies for not recognizing the bug earlier: I believe I've accidentally invalidated many reviews like that in the past months because I can do 20 reviews a day, but I have less than 20 delete votes a day, and as such potentially triggering the issue daily.