In watchOS 7, a grouped notification when viewed from the Notification Center no longer displays the usual snooze buttons at the bottom of the notification. This is a watchOS 7 bug which requires Apple to fix. We have already filed a bug report (FB8816479) since October 2020. Unfortunately Apple has not fixed the bug as of watchOS 7.6.
In Due 21.5 (released June 3, 2021), we worked around this problem by not grouping Due notifications on watchOS. Although this would take one more tap (compared with watchOS 6) to get to the mark done and snooze notification actions, this would at least allow you to get access to these notification action buttons.
The ideal fix would be for Apple to fix the bug in watchOS itself.