To shorten the trouble I had a look into JPG header description. It is like with nearly most image file formats that in the header only the pixel width and the pixel height is defined/saved/stored. So no DPI or paper scale description. Even for what cause it's to the user to define there to output and not job of the image.

I have checked with  lots of my Poser images (version 7, 8, 10) and camera images in some of the up to here often named image tools. Every time I open a picture I get 72 DPI even may be 96 DPI and have to set the print output scale thus adapting the DPI. This is normal behavior of most image programs cause running on the monitor which is using 72 or 96 DPI. So for me Poser is behaving normally and I don't know there it should store a DPI value in the image file. Ok, may be in EXIF data if storing such stuff. But EXIF data are not relevant for printer steering.

If you want to know more about JPG file header have a look here https://www.file-recovery.com/jpg-signature-format.htm

Or if you want to have a look to the header of one of your images try this https://cyber.meme.tips/jpdump/# there you get listed whats in your files header.