Trust me, I tried everything.Have you checked the notifications settings for the watch in the watch app? Also check the notification settings on your iPhone since there is a bug with iOS12 beta where by default Mail notifications are silent and or vibration is off. if you have your watch mirror your iPhone it could be because of this. Worth a try.
I have already opened bug report with Apple on this and support team reply fast. I already sent co-sysdiagnose. We will see...