I do wonder whether or not it has something to do on the user record itself as well, possibly?

I have to put in a caveat...it's typically only external users that have this issue.  If I send one in as an internal user (which all of our internal users have an @usbank.com suffix) it seems to work just fine.  not sure if it has something to do with Notification Devices, etc.  Just another food for thought 🙂