Reimported PNG looks great (until I press Save)

I’m seeing some weird behavior on one of my images, which looks fine in Photoshop, and looks fine in the Editor window on Reimport, but when I press Save it fouls up the pixel-perfect appearance. I’m assuming it has something to do with an applied Compression setting, but unfortunately there is no option for “none” on the right hand panel, and the others change the colors altogether. Anyone have any ideas about this? Thanks!