Based on the icon, the “whole mask” operator is “screen”, not “difference”. Click on it and change it to “difference”.
EDIT: I understand what is the source of the confusion. I see that you set the “blend mode” of the mask to difference:
That is a completely different dimension. The “blend mode” is the operator used to combine the output of the module with its input to produce the input for the next module in the pipe. It has no relation with how the mask is composed. And for the record, this is not a distinction introduced by the new masking panel, the blend mode has always been there, exactly in that position.