Hi. It's difficult to provide correct steps.
User3 get notification about allready deleted video.
Notification is about: "User1 commented user2's video".
I don't know how exactly video was deleted. May be user2 was deleted from the site by admin.

Link to notification: https://crimson-land.ru/community/notifications/route/126075

Anyway I suppose such notification should be deleted too?

Thanks.