Level display order not honoured when referenced

I'm having an issue where the display order priority of elements in a model is not being honoured when that model is referenced into another file. In my model the elements all appear as they should, like this:

. This includes the boundary around the sign detail being filled. When I reference that into a drawing it comes out looking like this:

The levels are all present and selectable in the drawing, but the level display priority order isn't being honoured so the boundary hatch is appearing in front of nearly all of the other elements. The display order is correct in my model, with the sign elements as priority 500 to make sure they appear on top and the boundary fill as priority 0, but it doesn't appear like that in my drawing. Can anyone help please, as I'm going spare trying to figure this one out!

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