Before I posted I tried restarting both the watch and the phone, unpairing the watch/re-pairing the watch restoring, unpairing the watch/re-pairing the watch as a new watch, and deleting the mail accounts and mail app and reinstalling/re configuring. None of this worked. All other notifications work as expected.
I should note that I regularly support this equipment as an IT professional.
I did get notifications to work for mail by turning on banners for Mail app on the phone (I really don't want this on) but audio only, no haptic. Also - Even with banner turned on, I get no mail notification for any mail if I chose Custom in the watch app to exclude one mailbox. In my opinion, this seems to be a bug in iOS 12, the mail app, or WatchOS 5 but I sure don't see many people complaining. Searching the web shows similar issues throughout the beta for iOS 12 and WatchOS 5.