I don't know if it's a missing feature, or a bug. But when someone offers a bounty on someone else's post, you cannot tag them in a comment using @.
Same goes when someone edits a post that is not theirs and they don't leave a comment.
I've asked on MSE's chat and was linked to this MSE post.
You can tag anyone in the revision list (reviewers excluded), but there will be no autocomplete (username needs to be typed manually/pasted).
That applies to editors and those who offer bounties.