underlay action

preset actions:
1. scale+crop to 130x130
2. an underlay image background of 150x150 (centered, maximum) - transparent edges
3. an overlay image to frame the photo, leaving the above transparent edges (no straight lines...).

At times the generated image comes out beautifully as expected, at times the transparent edges become black.
what could be the reason? Is it a bug? How to solve?