I reproduce the steps in Ubuntu 20 with Microsoft Teams version 1.4.00.13653(64-bit) and the result is the same.
The workaround is to use Teams web client(https://teams.microsoft.com) in Firefox.
This issue happens several months ago and still has no progress now. I am paying attention to the problem and if there is any update, I will share with you as soon as possible.
Meanwhile, to get a more efficient support, it is suggested to open a service request by this link. The support engineer will collect your backend log file and analyze this problem exactly.
Thanks for your patience and understanding.
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