As Lionhearted stated, you might have the silent timer checked. If not, it sounds like a UI bug with the OS as you see them there, but they don't pop up. Probably a rare edge case with a crapton of notifications coming through. When searching google with your issue, there's a lot of issues with the notifications that randomly stop.
If you don't want to rely on browser based notifications that link with the OS's native notification system, give https://forums.d2jsp.org/chat.php