You are correct... The shadow line is created by the scan overlap between the side elements and the down element. It is a known side effect of having separate elements in a sidescan transducer. Early models without a dedicated downscan element (which used the overlap of the two sidescan elements to produce a "down scan" image) did not display this same interference pattern (early HB for example) but suffered from less than clear downscan results.
Bottomline...that isn't a creek channel that follows your boat everywhere you go <LOL>...!