I have the same problem, the following helps a bit;

in mix options set the mix background to black (unless of course you are using multiple mixes on top of one another

if you have a keystone mix go to the options tab on the side bar and make sure the background is also set to black

this should reduce the dip a bit.

I assume you are fading one layer on top of another and then removing the bottom layer (or vice versa)? and not doing a straight crossfade?

I believe this problem only exists when going via a mix.
I know Richard is aware of it and is working on it.

Paul