When using Adobe Rush, my overlaid PNG image is shown with a black shadow bordering it. Remove?

Sure, thank you!

the first image is the PNG in Premiere Rush, which should ask as an mask/overlay and to blend in with the background image. It was painted over with color references, so I know the colors match and fade correctly. 

The second image is how they look layered, with the black shadow surrounding the PNG. I hope this is clear, please let me know if you have any more questions, and very grateful for your help!

I did try troubleshooting with a PNG file square, full transparency, no halftones or fade in the opacity, since my first thought was that it doesn't support halftones for some reason. The same black shadow is visible around the boarder, I tried experimenting with fades in the program, but they only effect one side, black shadow still showing on other sides of the PNG. 

Thanks so much for any help or advice!