FCP5 composite modes dont work with certain disolves

i dont think this is just me:

I'm creating a soft vignette on a clip
i've' got a "Shape" generator sitting on top of the clip, with the opacity down, and composite mode set to "multiply"

when i use a normal dissolve, everything is as expected.
when i use a fade in- fade out dissolve,
as soon as the transition starts, the opacity jumps up to 100%

other dissolves that dont work are:
additive dissolve
non-additive dissolve

nick