I am not familiar with “negative” features on Gerber files (don’t know much about Gerber at all, just the basics).
As far as I know Gerbview works very nicely with Gerbers generated by KiCad, but KiCad probably does not use those “negative features”.
My common sense tells me that the printout should look the same as in Gerview itself, so this sure does look like a bug in KiCad.
As far as I know KiCad developers are very keen on squashing any bug in KiCad, and a test file to reproduce a bug is a big step in improving the software.
I understand you want to be careful with the files of your customer. and the simplest solution is to just use another viewer.
If you want to help improving KiCad, there are a few simple things you can consider:
- You could share a single layer of the gerber confidentially with the kicad developers.
- You can delete 90% or more from a gerber layer and upload it here.
- You can ask your customer to make a small test file with some “negative” features.
Do you know with which software those gerbers were generated?
If you do, then maybe another KiCad user with access to that software can make a test file.