Hello. I strongly recommend High priority, despite it is not page content lost, but still, the user loses a lot of information.

1. User A has page X in their watchlist.
2. There are a couple (one is enough, but let's say 5) unseen revisions of X.
3. Before revision 1 there was their own revision of X.
4. User C thanks to user A for that revision.
5. User A gets notification about this thank.
6. User A clicks on notification and opens the thanked revision.
7. Unexpected result: Revisions 1-5 marked as seen.

Recognized many times, including five minutes ago. But at least once it worked as expected.
Thank you.