But my question now is, what is the point of this Res: setting when, if set too low reduces the size of the print.
If you use Custom, Use Original size, it's possible to override the PPI by ticking the box and setting a PPI, so make that is not ticked.
What PPI you read at the bottom of the screen is the embedded information of the image, if it's not what you expect, it's the image data not QU.
The actual print size sent to the printer from Q can be seen in the Queue tab rather than using the page editor. With an embedded size and QU set to use original Size, that will show the PPI too. I checked one here and all is as expected, both sets of numbers agree.
If linear dimensions are set (not original size), the PPI shown in the Queue tab is that calculated from the image pixel size and the print size. Of course, QU interpolated to the printer native resolution regardless so I hope you are not interpolating (re-sizing) in PS because will degrade the final print quality.
Personally I think it's a waste of time "sizing" an image in PS when QU overcomes all that procedure and associated opportunity for errors by simply allowing you to use the original image file (after edits if required) and specifying the linear dimensions in inches or mm. and then, with crop scissors on, adjusting the crop as you wish. You don't have to bother about PPI etc. as QU does that for you. After all, it is one of the main features of Qimage.
Terry