Fill error in hotlink

[Anonymous]

Hi,
I'm trying to resolve an error which removes background fill when I'm hotlinking a detail.
I have drawn a bunch of details in separate files and saved them as PLN (and MOD for testing purposes). Many of these details have fills with the 'Fill Pen: Background' as '(-1) Window Background'. These display properly in the original file and cover up unwanted detail behind them.
In my job file, I hotlink the drawings on to new detail sheets in the project map and suddenly I lose all the masking from the fills and I can see all the lines that were previously hidden.

I've noticed that if I place the detail on the ground floor, this problem doesn't exist. So I assume it is something to do with the detail pages I have created but can't find a solution.

Any help is appreciated.

Cheers!