The color is 8-bit depth, and all 4 channels (RGBA) are active. But I have since moved on because I didn't realize there was a response here
Have you been able to reproduce this problem?
I'm having the same problem resizing PNG's. The alpha layer just turns black for any resample type other than "bilinear" or "nearest neighbor" (and those produce pixelated images that don't look good). I tried enabling/disabling the "Option/Read/PNG/Compose ...." in every combination. I can't even ...