auto transparency
 

Any black image that is completely black will go into transparent mode x.x

Nothing new for me, it's always done that for a long time, pre-v5.

:o

Maybe I should elaborate.
In Graal, pure white(255,255,255) and pure black (0,0,0) are reserved for transparency. This is why I never use those colors, and use near-white/black. But... sometimes it wouldn't work, as many use those two colors, but I could never get it to ignore the transparency.
Either way, the feature is outdated and should be removed. No one uses it on purpose, and just 'causes problems. I say if it isn't going to be removed, have it as some really obscure color like neon magenta or something like most use.

For gifs it is using black as transparent color if there is no transparent color specified in the image file, this is to stay compatible. For other images it is not doing any transparency if the file has no transparency. For images with a transparent color index it is setting transparency to all pixels that use the same color as the transparent color index, although there seem to be some problems with that in v5 (see the head transparency problem on Kingdoms).

It's only happening on .PNG images for me.

Can you post the png here so that I can check it?

It's really any png image that I've saved with Paint.NET
I can open it & re-save it with IrfanView and it dosen't do the auto-trans thing.