It’s a bug that’s been around since watchOS 1.0.
If you notice, the notification mail icon is round if it’s working correctly, but square if it’s a generic app icon (no ability to delete, mark as unread, etc.).
Generally, a restart of phone and watch fixes it. Opening the email app on the unlocked phone then opening the email app on the watch may help.
What seems to happen is that the watch loses connection to the system email app (the watch’s email list won’t be up to date) but the generic notifications come in.
Look for the rounds versus square email icon in the notification. That’ll tell you if you’re experiencing this issue.