Some apps will update their notifications instead of emitting new ones. This updates are not shown on the desktop. 

Example: The Signal messenger does show a notification on message arrival (something like "ContactX:prefix of message..." for the first message). When a second message is received before the first one was read (i.e. notification is removed), the notification will update and show a summary (something like "there are 2 unread messages. ContactX:prefix of message..., ContactY:prefix of message..."). This change is never shown by notification sync.

Reproducible: Always