This is perhaps more of a PSA than a question, since the workaround is just to produce the pngs outside of bridge, which has a native function to batch save psds as pngs. Photoshop can do the same it seems, I just need to create an action, since the image processor in PS only allows tiffs and jpgs.
So, this wasn't happening with every png, but it was happening with a lot of them. I tried the usual troubleshooting, changing various rendering settings and exporting expressly without transparency, and then eventually figured it could be the images themselves. They were being rendered with a massively reduced bit depth and just wrong, most of this should be light grey.
Bridge is on version 18.104.22.168, and this is Premiere 15.2.0. Always a shame when the suite does the opposite of working together.