[idvusers] "Visible range" not showing image beneath

I've got a sat pic (from a server under "Images") on the surface, and
a colour-shaded plot of theta-e at 850 hPa from the GFS above. I
restrict the visible range of the theta-e to a value so only areas
above a certain value are shown (to illustrate a plume event). However
areas with theta-e below my range appear black, rather than showing
through the sat pic below.

Am I missing something simple, is this is intended behaviour, or is it a bug?