I think the number of notifications should disappear once I click to see them, weather I’ve opened them or not should make no difference.
For the convenience, I can explain the psychological dilemma that arises due to the current behavior.
If the team is not convinced yet, then think about Facebook, Google, LinkedIn and all of these big names. For example you got +99 Gmail notifications, the counter will reset once you click even if the page nesting allows only 10 notifications to appear at once.