Mask feather becomes visible only upon export - how to fix?

[Rob Burgundy]

Hey yall, I am experiencing some issues using masks for Lumetri...as long as im in Premiere everything looks fine but when I go to render my file i can clearly see artefacts of the mask feather appearing. Is there any way to get rid of these? I have tried adjusting mask feather and expansion but everytime i go to export i can see the artefacts in the preview window. 

Inside of premiere:

Media encoder preview:

Please advise! 

[Ann Bens]

Add a slight blur but with an inverted mask

[Rob Burgundy]

Thanks, i had tried this before but will give it a more in-depth try now. When you say slight blur, what kind of value are you thinking of? Like somewhere between 15-50? 

[Ann Bens]

Start with Gaussian Blur 1