I was flagging some "no longer needed" comments on a migrated question and in some circumstances which I haven't been able to narrow down an extraneous red flag was showing on a comment further down my screen in addition to showing correctly on the comment I was flagging. It wasn't the one right after the comment I was flagging.
I'm browsing in Safari 14.1/iOS 14.7 on my phone using the responsive full site view. Reloading the page fixes the display of the flagged comments. I had the comment list expanded. I was able to reproduce it a few times, but I'm not sure which steps would cause it to happen every time (and I've run out of comments to flag legitimately).
I'm having trouble reproducing this while paying attention to what I'm doing, so I think it's likely that it's something related to me shifting things around or selecting things while dragging the flag dialog around at some arbitrary zoom level.
I caught it happening as I was flagging the second comment as "no longer needed". You can see the light blue square around the flag that I had selected. I did not flag the third comment. I did flag too fast and had to dismiss the warning (I have to do that almost every time I'm flagging comments as "no longer needed", so it could be a factor). Refreshing the page makes the flag go back to unhighlighted.
(And there's nothing wrong with these flagged comments; the author updated their question, so they served their purpose.)
Previously, there was a comment that didn't have the flag highlighted between the comment I was flagging and the erroneously highlighted one, so I have a feeling this has to do with the positioning of the comment on the visible portion of the page, not its order in the list.