I'm using the QC 35 wireless Bluetooth headset (I think). With no apparent cause, my sidetone is suddenly lagging by a sizable fraction of a second, so any time I talk or some other noise is made in my vicinity, I hear it as an echo. This only happens in connection with my laptop running Windows 10.
I solved the issue, which is not related to the model of headphones used. It's the effect of the "listen" checkbox in any microphone/headset's details, when visiting the Recording tab of the audio devices subsection of the Windows control panel.