It doesn't visually change only because you insist on setting up ProPHoto RGB as a soft proof and the data you see is basically in ProPhoto RGB! If you select another color space, the preview will very likely change because you are far farther from the original and editing that data must and should be done on a VC, NOT THE MASTER!
The data and histogram I see without Soft Proof on is reflecting output of what? Linear with all the Adobe LR default tones applied which would explain why the histogram isn't slammed to the left on a new Raw master in Develop module? Is that correct?
The thing about LR is that images and their edits are in a constant state of perpetuity until the edits are committed to export to a tiff or jpeg so while in LR
with Soft Proof turned off the data reflected in the histogram and "%" numbers have very little connection to what's reflected in the preview with regard to measuring and defining the preview through HSL or RGB analysis. Do I have that correct?
I never tried changing Soft Proof setting from ProPhotoRGB to another matrix profile to test to see a preview change, but if I understand you right (despite my doubts I'm able to), if I decide to Soft Proof to a printer profile with edits made using Soft Proof set to ProPhotoRGB on the "Make This The Proof" (I assume this means the master IS the proof) selection, the printer Soft Proof will only reflect the edits applied/encoded on the original master
when Soft Proof was off and reflecting Linear with Adobe Default settings applied. Is that right?
I've never printed out of LR, yet, but I can't Soft Proof my printer anyway due to the fact I use Printer Manages Color for my little Epson All In One. So to make this less complicated and more manageable, I'm assuming I shouldn't even turn Soft Proof on to reflect ProPhotoRGB output working on the master and not a VC-(I really don't like making VC's because it creates duplicate thumbnails in Library Grid mode as mentioned above).
I'm having trouble remembering and sorting out in Library Grid mode which are the master and VC or Proof Copy even though I know the icon for VC is the turned page corner in the lower left of the thumbnail. I don't know if that's a Proof Copy VC or just a plain VC and if there is a difference. It turns into a rats nest of duplicate thumbnail previews where you have to look very carefully at those tiny icons.
I'm trying to make LR work like ACR with regard to workflow and thumbnail managing in Library mode and this Soft Proofing thingy on a master or VC or Proof Copy is just too much to rap my head around and keep organized in my head. I'm never going to print out of LR or need Soft Proofing to a printer anyway.
I should keep Soft Proof off if I'm working on the master file if I want to make LR work like ACR.