Turns out that it only happens on Bluetooth, when two devices are connected to the headphones. (The other device isn't playing anything)
Unfortunately the pattern is not clear. I had the pattern reverse. Though the trigger seems to be related to the change of headphone connection status to a secondary device.
Thanks for the report - sorry you're having issues! This might be a tricky one to work out unless we're able to find a set of steps that can reproduce the issue on a device that I have - if it is specific to your setup, it'll be hard for me to test things to see what is causing it :(