Variants, assembly drawings, red crosses and mechanical layer 16

[jeroen74]

 I have a design with SMD components on both sides, and I have defined a variant with several components that are not fitted. I set the variant drawing style for the PCB drawing to a red cross; and for some reason the generated assembly drawings for the top and bottom sides both have the crosses for both component sides, instead of the top assembly drawing just having the crosses for non-fitted components on the top side (and the same for the bottom view)

I happen to use mechanical layer 16 for the board outline, and the strange thing is that this layer causes the problem. If I use any other layer the problem disappears. Just one track on M16 is enough for Altium to draw red crosses on that layer in the printout for both sides.

Mechanical pairs are defined for 13-12, 15-18,25-26 and 30-31 (respectively 3D bodies, outlines, assembly and page borders)

Is there anything special about mechanical layer 16? Just Altium weirdness?