Hi,
I have had this issue quite a few times.
Unfortunately, I can't provide any fix for it as the only method I have of getting it working again is uninstalling and reinstalling the app. However, one method I just thought of is possibly signing out and back into the app.
I believe the bug that makes it not appear is when you miss the IOS Prompt that appears when you sign into an application that requests to send notifications to your device. I believe that if you don't respond to the prompt, then it doesn't store any notification setting for that application - therefore a simple reinstall can fix the issue if you ignore the prompt.
Hope this helps