We solved a lot of issues with the combination of blend modes / surface opacity / media alpha in MadMapper 4.0. In 3.7.5, if you chose "Stencil Luma" the opacity was ignored. That is now solved. I suppose that's your problem.
If it's not, just send a minimal project file (together with the images in a ZIP) we'll have a look. But I just checked again and it does what it should.
Are you using an FX ?