Hello Ling Yan Tung,
Good to see you again in Microsoft Community.
I tried to reproduce the scenario you described and found the same problem, but I found an alternative.
You can install Microsoft-released PowerToys(Microsoft PowerToys - Microsoft Store Apps) in the Microsoft Store, which has the globally muted shortcut enabled, and you can map that shortcut in your 4-finger tap custom settings in Windows Settings. Visit PowerToys Video Conference Mute utility for Windows | Microsoft Learn for more details.
If anything is unclear, please do not hesitate to let me know.
Best Regards,
Mosken_L - MSFT | Microsoft Community Support Specialist