Additional team and channel-related features and issues within Microsoft Teams for Education
Hi Heinz,
Please tell us if you follow the steps in this article to create a new assignment in Microsoft Teams: https://support.office.com/en-us/article/Create-an-assignment-in-Microsoft-Teams-23c128d0-ec34-4691-9511-661fba8599be.
To detect if the problem is related to your Office 365 account or not, we recommend that you invite any other Microsoft Teams owner to create a test assignment and check if the problem persists. Also, try using Microsoft Teams web application to see if it helps.
If you add the members after you created the assignment, they maybe not able to see the previous assignment. Try creating a new assignment and check if they can see the new one .After you get the replies from these users, you can just delete the new Microsoft Teams and just use the original Microsoft Teams.
Thanks,
Huni