PSP X4 has started opening JPGs in different size and resolution ...?

[Firefox1701]

I'm pretty sure this has just started happening, although I'm not what you'd call an intensive user of Paintshop. I've got a PSP image saved at 100 pixels per centimetre, sized at 290.4mm x 204.8mm. If I merge all the layers and then save it as a .jpg, without changing any of the above, it appears to save it at the same size and resolution - which is to say, if I go to 'current image information' with the file still open, it says it's saved it that way. However, if I then close the file ( or even close Paintshop altogether ) and then re-open it, it opens at 72 pixels per inch, 40.333 inches x 28.444 inches. For the life of me I can't figure out why. I'm by no means an expert either with Paintshop or with computers in general, but I'm also not a complete dufus, and I can't see what I'm doing or what I could change or do differently that's causing it to behave this way. Somebody please help me!

[LeviFiction]

PSP, if it can't find the physical size data in EXIF it'll use it's own defaults for this virtual data. Can't say that's what's happening here but it is something we can check. Do you have JPEG set to save EXIF data? File -> Save As -> Choose JPG from the file format and hit the "Options" button in the bottom corner. There is a checkbox for saving EXIF data.

[Firefox1701]

This did seem promising, but upon checking, the box in question is already ticked. Are there any other things in that 'options' window that I need to look at? It's currently set for 'lowest compression - best quality'. Failing that, any other ideas?

[Firefox1701]

By way of a postscript, I've just checked the EXIF information for the image I'm testing this on, and it's showing 100dpcm for both the x and y resolution ( which is what I last saved it as ), although the Image Information shows 72 pixels per inch.

[LeviFiction]

Awesome then it's writing it correctly, it's just not reading it in properly. I have no solution for that. But a workaround is to change the resolution back after opening the image. It's the best I've got.

[Firefox1701]

I've been doing that in the absence of any better solution, but the files that have brought this to my notice are book covers involving a significant amount of text, and I find that each time I change the resolution there seems to be a ( very slight, admittedly ) loss of sharpness in the text. I say 'very slight'; obviously this becomes progressively more noticeable when you've closed and re-opened the file several times.
I appreciate your input; any other thoughts or suggestions gratefully received.
This still wouldn't be ideal, but continuing on the above theme, if I can't find any means of getting the programme to open the files at the resolution at which they were saved, as an alternative, is there any way to change the default EXIF settings?

[LeviFiction]

1) Don't save as JPG until you're done editing. Save as PSPImage. JPEG should be a final format only. My guess is it's a mix of resampling and saving to a lossy format like JPEG.
2) Try turning off "resample". And always make sure you've set the percentage to 100% when changing the print size.
3) You can use tools like ExifTool to change resolutions without opening and resaving in PSP.
ExifTool Link - https://exiftool.org/
Demonstration - https://www.youtube.com/watch?v=bUwAN7mIVKc