- load a video clip;
- assign the blur effect to that clip; the clip will be blurred;
- then open the mask designer and apply the (new) custom selection mask;
- after OK the mask does not show in the preview screen;
- if you switch off the blur; the mask will be revealed;
- if you have the blur mask in combination with another effect, depending on the order of the effects, the mask will be visible or not
When I use one of the predefined masks in step 3 above, this mask will show as expected.
I expect that this behavior has to do with the fact that the blur effect also uses masking and that the custom made masker does not completely behave like the predefined masks (yet?).
Maybe it is meant to be, maybe not. I wonder whether you have the same observations.