Dave is not the only person experiencing this problem. I have had it for a week or so also. Though I'm using a Plantronics Savi W710, so it's not a specific issue with his specific headset model. I've had to resort to using the on-screen mute button instead,
which does still correctly drive the plantronics headset.
If I push the headset mute button while all is unmuted, both the plantronics headset base and the teams app toggle the mute status, but then a second or two later the headset base unmutes (but not the teams app). After that, if I push the headset mute button,
both the app and base mute status toggles, and then the headset mute status toggles back. But sometimes it doesn't. It seems a little bit random.