I ecountered the same problem I always had with IrfanView used portable before it came out in PAc-format. I just didn't remember the issue before.
When switching through options to change to preferred settings, irfanview portable hijacks file associations for images if I don't remeber to uncheck that. I was confused when all my settings from my local installation were gone, but then realized that Irfanview was started from my still connected USB-drive (bypassing its launcher as well ).
When I unplugged it, all associations were gone. I had to start IrfanView manually and check options, then all was fine again as IrfanView restored its associations from its local settings file.
It's not a real problem when you know what to do, but I consider it a design flaw. Maybe it is possible to backup the local settings first and then switch back after the PA.c Version quits.
Unchecking is no remedy. And associations are not gone but unfunctional after unplugging. (See post below).
As long as I don't change settings, all is fine, but that cripples IrfanView a bit as e.g. Fullscreen options are done with the same dialog-box.