Looks like I was a bit premature in calling this sorted.
Using pitch reduce (either slow or full-speed) causes prop desync. Usually, but not always, it registers a button press and reduces the stbd engine, but not port. If you press and hold, stbd will get a headstart, then they move together. I’ve been able to manage to get the stbd lever to move at twice the speed as the port, using the right/wrong frequency of button presses.
What is interesting about this bug is two things:
1: it only affects prop pitch decrease. Increasing prop pitch works as expected, as does prop mix up/down.
2: it is always the stbd engine that gets a greater decrease (you know what I mean), never the port.