I haven't researched this in detail yet, but there seems to be a difference in the shape of notifications for PRs and for discussions.

Earlier today I had three notifications in my inbox, 2 from discussions and 1 from a PR. When trying to fetch all notifications I received this error:

ParseError "Error in $[0].subject.url: parsing URL failed, expected String, but encountered Null"

I marked one of the discussion notifications as done, ran my code again and got the same parse error. Then I marked the second discussion notification as done, and when I ran my code after that it happily parsed the response with a single PR notification.