Is it because you have the following option enabled?
Show Edge lighting -> While screen is off
"The tip under that option says Edge lighting will replace notifications that turn the screen on"
This may only be the reason if you have that option enabled and you have WhatsApp selected as one of the apps that utilises Edge lighting. This is one of the challenges Samsung faces in my opinion, there's far too many options and some of these options have adverse effects that aren't clearly described.
I'm come from a Pixel 4 and I'm loving the engineering behind this foldable but I can't stand Samsung's UI. It's too messy and very unprofessional looking (hate the rounded corners).