In the same sitting, I’d made two more images - some text and a logo in the corner, and everything else was just transparency. The logo had semitransparent parts, but no gradients. This displayed fine, including the semitransparent parts.
It’s only on gradients that it seems to stumble. I don’t know how PNGs store their gradients, but I know it’s not the file format that’s to blame: Both Photoshop and Gimp can open each other’s files, and they display fine - a white gradient on a transparent background.
The 'Add’ing is the thing that confuses me. I can think of no reason for it to do that.
I’m sure it’s being added, as when I replace the white gradient with a black one, it displays fine. As soon as I start adding colour, it goes to hell.
But does noone else get this? If you make a PNG with a gradient, put it over another image in the Sequencer, and set it to ‘Alpha Over’, what do you see?