Basically, if the microphone is not muted I can always hear what it records. Even when I do nothing or play some music. Windows "Listening" mode is disabled, if enable it I can hear an additional echo. Sidetone volume reacts to the setting in the Plantronics Hub (Low/Medium/High), so it's definitely not an external issue. I do not mind to hear it during the call, but it's not just annoying (I can mute it, in any case) - it drains the battery all the time if headset is not powered off. Before the problem appeared I have been charging the headset about once per week, now I have to do it every day.
I am using the provided BT700 adapter. Firmware is the latest (647-2725), and I suspect that this firmware could have caused the problem. Also, somehow the issue doesn't reproduce on Mac Monterey, but reproduces on two different Windows 10 laptops.