Disappearing email notifications in watchOS 5

Since I upgraded my Series 1 Watch to OS 5, my email notifications always disappear as soon as they arrive. I get the vibration telling me I have a new email, but when I check the Watch, there is no red dot, no message in the Notifications screen. The email remains unread on all my devices. If I check the Watch immediately after the vibration, I will see the new message appear on screen, as has always been the case. Any delay, though, and all evidence of its arrival vanishes (though if I go to the Mail app on the Watch, the new message is there). Notifications for texts and other alerts all appear to be working normally. I haven’t changed the Mail notification settings — they’ve always mirrored the phone. Anybody else seeing this?