Looks like the author has deleted that status after FE has already received and stored the corresponding notification (because of the typo I guess, since their next status is the corrected version of it). Since the notification no longer exists (BE deletes it along with the status), the FE most likely gets the 4xx error when calling POST /api/v1/pleroma/notifications/read.
E.g., steps to reproduce:
mention the user while their FE is opened but the window is unfocused (so that the notification is not read automatically, as soon as it appears)
wait a few seconds for the notification to be created and added to FE
delete the status
as the receiving user, open the notification page, FE tries to read it but gets the 400 response since it no longer exists on BE