It seems there is a bug with the UI around dismissing notifications from the Vivoactive 5.
(Anyone else seeing this? Is there a better place to report this bug?)
When on the notification page on the watch, a swipe-left on a message reveals a dismissal UI (red X). Pressing that X dismisses the notification. This works as expected for the first message dismissed.
Without navigating away from the notification list, subsequent swipe-left gestures have no visual impact on the message. The message can still be dismissed by tapping where the red X would have appeared, so this seems to a presentation issue, not functionality.