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Yes that is the behavior of the app badge. I believe this is because app badges are driven by the notifications. Looking at it in reverse, I cannot think of a single app that I've used that keeps the badge count if I swipe the notification away. Sorry but that's just the way it is. If you want to change the app badge count clear the notification shade.
While I can't speak for the S9 but from the S10 to now this has been the case. I'm pretty sure this was the case for the HTC phones I used before moving to a Samsung device. I'm a bit surprised that your notification shade doesn't consolidate the notifications into one notification but that may be app dependent. For instance, I use Tapatalk to access this forum and it will keep the notifications separate until I get to around 5 or perhaps it is until a certain amount of time has passed, either way, they then become one notification that I can just swipe away.
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