Thanks for the response. Of course after weeks of seeing these, I made some progress on my own after posting. It looks like the trouble is .emf file images. Most of the icons are .png, but the troublesome ones are .emf. Doesn't look like
draw.io knows what to do with them, so I get these crossed-out camera images instead.
I thought it might be
draw.io on MacOS thing, but I'm getting the same on Windows.
Guess I need to unzip the .vssx, convert .emf to .png files on my own, figure out where the .emf files are referenced elsewhere in the bundle and change the filenames to .png versions, then zip it all back up... What could go wrong?