Andy’s comments started me wondering about this. There are two ways to move the spectrum vertical position in the camera field. You can adjust the spectrograph mirror or you can move the star along the slit. You commented, Kevin that the slit position and the sweet spot did not correspond to the centre of the guider field. (They should do when everything is correct.) I wonder if because of this, where you are placing the star is off the spectrograph axis, exaggerating the widening effect in the spectrum.
Robin