What would expect this to do? Should it remove the line separator, or should it preserve it and then just not fail?

I think the formatter should not remove anything when it fails, but report the line and character position, so the developer can remove it themselves.

If the guard is specifically about a Line Separator character in comments (which I consider an invisible/hidden character) then I would expect it being removed without failure.

Edit: I re-read what I just wrote and it may sound like a contradiction, but what I meant is that I expect the LS in comments to be removed in my case, while for any other failure I expect, if possible, a line/character position of the fail.

what I meant is that I expect the LS in comments to be removed in my case

That sounds good to me too. The failure mode it's getting into now is that it does remove it, but then the sanity check it runs to make sure it did not make any incorrect changes does not expect to see that character removed, so complains. I'll fix that so that it allows the line separator to be stripped.