Overbridge mode seems to do nothing?

Got a second hand Heat mk1 today, tried setting up Overbridge and it was a bit buggy so updated from 1.02 to 1.20, it’s now as if the unit isn’t even connected when it’s in Overbridge mode.

It shows up as a midi device for the other two settings, and my Digitakt works just fine using the same PC and USB cable - is it likely to be a fault with the Heat itself?