I often observe a behaviour that looks like a bug, although I can explain it.
For example, if I get two likes on a post, the counter goes up by 2, but only one notification is shown (which makes sense). Now when I click on that notification, the counter drops by 1 and the notification is no longer highlighted. This causes a discrepancy between the counter and the highlighted notifications, which can only be resolved by clicking on the small check mark in the upper right corner.
I would expect the counter to decrease by two when I click on a notification that represents two events.