Additional assignment-related features and issues within Microsoft Teams for Education
This is from a user on Reddit but it worked for me :)
Please do try and follow the steps below along with me:
Go to the Assignments tab where the red dot is shown and then on the topmost of the page there is something called "Search or type command" click on that bar and type "Assignments", then you'll see an app for assignment with a purple-coloured icon, click on that and you'll get your unread assignments mentioned. It usually comes when the person who sent the assignment returns back a turned in assignment. Hope this helped!