I created the entire model from scratch. I then geolocated it by putting in the co-ordinates directly. It created the background map photo as I would expect but then I turned on shadows and started to notice the clipping. The only reason I mention it is because it sounds remarkably similar to the issues mentioned here: Bizarre behavior of shadows - #3 by mrwmrutski
I’m as sure as I can reasonably be that I didn’t do anything to put that extra slice of google imagery up there - I didn’t manipulate the background directly at all. My impression is that there may be a particular set of circumstances under which adding a location does something odd; it’s quite possible that my model was not configured in an optimal fashion - I’m not an especially experienced user of the tool - but I did very little between adding the location for the first time and the problem showing up, so I’m not really sure how I could have done it inadvertently…