“I have two suggestions. First make sure your blur is set to repeat edge pixels.”
Where is that option? I can’t see it in the gaussian blur controls.
“If that doesn’t help, apply a levels effect after the blur and drop down to affect the alpha channel. Set the white input point on the alpha channel to 0.5 (or about 127 if you’re using 8bpc numbers)”
If I would do that it would completely change my mask, making it harsher again.