This just happened to me on fawiki.
It was not my edit that was reverted (and indeed, I did not even edit the page, I just protected it). I am afraid that User A never received the notification either.
Could you tell me which edit this was, so that I can verify whether user A received the notification?
I suspect that what's happening here is that the revert was treated as a revert of multiple edits, in which case both you and user A should have gotten notified. (Skipping over no-op edits like page protections would be nice though.)
I found it, and you're right that only you were notified, user A was not. It looks like every undo is only recognized as an undo of one revision, and somehow this one was interpreted as an undo of your revision instead of the one it actually undid.
I think what happened is that user B did a range undo: they viewed a multi-revision diff using the radio buttons on the history page (or were taken to a multi-revision diff from a watchlist email), then clicked the undo link on the diff page. That takes them to a multi-revision undo, but there's a bug where those only notify the author of the newest revision being undone, while not notifying any of the other authors.
So, strange as it may sound, this bug is a duplicate of T153570: Notify users when undoing multiple revisions