I don't understand why MyTraffic took priority as it is much further down in the scenery than MDPC. Any thoughts? I load MyTraffic as an addon in add-on.xml rather than through a direct entry in scenery.cfg.
That's why. The ones in the add-on.xml are always being read first by GSX.
The current version of GSX recognize areas added with the add-on.xml method, it still doesn't support Layers across the add-on.xml and the scenery.cfg as a whole, so it reads the ones in the add-on.xml first.
In any case, it's always best to delete the MyTraffic .BGL, if have an addon airport, because they used to cause problems like double entries even before the add-on.xml method existed.