I've had the above visual error when playing on No Mercy for many weeks now, and I've only just determined that this error only arises whenever I have 'City 5 part3' enabled in my Add-ons, and it's therefore been responsible for the error this whole time.

Obviously I didn't naturally make this connection sooner as The Parish typically has nothing to do with No Mercy, but I'm guessing there are certain attributes shared between these maps and hence the connection here regarding this error.

Anyway, just putting the word out in case anybody else benefits from this info.