I have the same question and the same trouble.

I can not use the voice button on bluetooth headsets (Jabra Stealth and Jabra Elite 65t ) too. When I push the button nothing happens.
No Google Assistant and no other voice assistants are called.  I guess it is a common problem in Flymeos. Tried on Meizu 16 and Meizu 7 with Jabra Stealth and Jabra 65T. It is the pain and the shame for Flyme.

And there is no information about solution here and in the Internet.

So if you get solution, please, let me know...