Notification Center "Show Message Preview" doesn't work In Notifications Preferences, I have the "Show Message Preview" option disabled for Mail.app, but nonetheless, message previews still appear in Notification Center.

How can I fix this? 

macOS Sierra, Mail.app 10.3
The checkbox only affects the notifications when they display as a notification, not when they are subsequently shown in Notification Center. Notifications are always shown in full with the message preview when viewing them in Notification Center.