AAD Usertype Guest cant be added to Teams and cant see In-Meeting Chat or other In-Meeting Apps

Hello everyone,
I am using Teams for 6 years now and worked with Guest Users in my tenant extensively. 
6 weeks ago a Guest told me he could not see the Chat Icon in our (calendar) meeting. He could participate in the Meeting but not the chat.
I did not find a reason and gave him access to an account of Usertype Member with a Teams license.
After the meeting I did a lot of testing a saw that no Guest account in my tenant would see the Chat icon or any other In-Meeting App in Teams. I went through all settings I could think of or found using Google and ChatGPT. There was nothing that might prevent Guest Access. 
I then realised that when @mentioning someone in the Meeting-Chat there popped up an option to add the person to the chat. After I did this the Guest could see the ChatIcon and other In-Meeting Apps.
I opened a support ticket. The support member went through all settings with me again but didnt find a reason. She proposed some other settings and policy changings afterwards (which seemed pretty clueless to me) but nothing helped. I then realised that it was also not possible to add Guests to a Team anymore.
The ticket is open since a month now, and nothing helped. The support member once wrote that they are checking some things cause it may be "a problem on their side" without adding more detail. But it all didnt lead to anything helpful.
Do others face the same problem? Is there anyone out there who might have a more profound knowledge? Any advice to me?