Update: sent a message to the seller, haven't got a response back yet. But I can now reproduce the issue on the bench. Simply turning to full lock, waiting a moment, then releasing the wheel to get the servo to return to centre as fast as possible is enough to trigger it. Might be a brownout issue, but every other receiver I've used works fine on this car with this servo and ESC BEC. So maybe it's just more sensitive to brownouts. I'll try a glitch buster cap at some point, and once my pocket oscilloscope arrives I'll see if I can monitor the voltage rail.

https://www.youtube.com/watch?v=JPMEhGMm7yo