Using transparency in .psb files

Hey,

Recently I’ve been trying to create transparent image and import it in Unity in .psb format. Since Unity’s .psb importer doesn’t support opacity of layers, I decided to create opacity with mask. However, for some reason after importing there are two white borders at the bottom and right of the image.

These are screenshots from Photoshop and Unity game view. Could someone explain to me what’s happening here?

P.S.

Doing exactly the same things but in .psd works as expected (no weird borders), so seems it has something with .psb.