Hi Skybridge,
Based on testing, this problem appears to share the same root cause as this thread
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No Microsoft Teams Red Sharing Border (nor controls) on External Monitors
It seems the mini window does not appear on the external screen. As a workaround, can I suggest trying "Video call" or "Audio call" from a group chat? This opens "full window" which doesn't seem to be affected by this problem.
Regards,
Szymon