Yes, the problem is that the buildings the bankers are in are classed as placeable objects, rather than part of the ground mesh. The .MAP file data that BestZ uses doesn't contain information about the placeable objects.
You will find that if you try and cast a spell on something you can't see behind one of these buildings, it will work because the Line Of Sight code uses the same data.
EDIT: Setting Map:FixPathingZWhenLoading to false, and maybe also Map:FixPathingZOnSendTo and leaving the moving/waypoint BestZ options on should fix it if the mobs aren't on a grid.
The images below show the same scene first with the placeable objects, and then without.