This is a split off of T139147: Mark notifications as read/unread with blue dot does not work in some circumstances to specifically deal with this bug, which seems to be repeating itself, as reported by @IKhitron
To my understanding, the issue is that an unread notification is marked 'read', and then, a few hours later, may reappear as if it is unread again, without explicit action by the user (@IKhitron please correct this if I got it wrong)