Thanks for sticking with me on this, Rory.
I installed 10.0.0.4 and it's still behaving the same. It seems to happen most obviously using the "Asym" model under "Overdrive". Moving a hard-panned signal even slightly towards the center seems to add a lot of signal in the center that sounds almost detached or delayed from the side signal. Conversely, using the "DCAM>Transistor" model maintains the panning as expected. Perhaps it's a side-effect of certain models and isn't a bug?