Seems to have something to do with KSPs inability to find the center of mass of the vessel; likely due to the ISA model.... not having any actual model.

But really, no clue

Adding -any- other part to the vessel seems to fix the issue.

Is related to the part having a mass of zero; apparently I am not setting/not able to set the mass prior to it finding the COM. Have set a mass of 0.01 in the config and this appears to fix it. Found solution while same bug cropped up to a zero-mass resource-switch part.

Fixed in dev.
Available next update