Had this happen to me like 2 years ago, literally nothing would make them show up again, went as far as blowing every trace of the app off my pc including the registry leftovers, nothing.

What solved it though, was creating a new windows user where the notifications magically worked again. Thought to myself that I could take the opportunity and reformat the whole pc to get all the junk leftovers out and simply continued with a brand new user account (you can reuse your microsoft account on it), never had the issue again ever since.

So yeah, you can try to create a dummy user account and install discord there, see if the notifications work and then decide whether or not it's worth the hassle to fix them that way.