Hello @komatat ,
I did a complete analysis of the test cases you mentioned and looks like it's a Bug as of now in Teams app. Why I am sating it as a bug is, the team disappears from the app is presented in Teams Admin, Azure Admin and in PowerShell search.
So, I did some testing and found a way to resolve the issue. You can also adopt the approach until it is fixed by MS. There are many user complaints on the same topic already and hopefully, it will come as a fix asap as I guess
- Create a dummy user contact in the Office 365 portal
- When you see the issues in both cases, add the dummy user as a member to the teams from Teams admin portal. Make the dummy user as admin of the Teams. Change the dummy back to member. Quit Teams (Not close, make sure you quit) and re-open and you will see the team created back to the application. You can remove the dummy user from the team if needed.
Please mark as "Accept the answer" if the above steps helps you. Others with similar issues can also follow the solution as per your suggestion