I guess that tells you how many of us use Linux

If you remember when reading metadata was new, there were all sorts of problems depending on which software had written the file. I'm guessing you've found an app whose format is valid, but isn't properly read by the agencies because no one was using that before.

I don't (my Mac is Unix under the hood, but that doesn't count), but I did wonder if Irfanview was available on Linux. It isn't, but apparently runs OK under a Windows virtual machine, but I found a list of Irfanview alternatives for Linux and wondered if you'd tried any of these?

https://alternativeto.net/software/irfanview/alternatives/?platform=linux

There's also ExifTool - and if you scroll down there's a list of Linux tools that use this library

https://www.sno.phy.queensu.ca/~phil/exiftool/