I don't see any issue with printing from a proof copy. In fact Step 3 in the referred document says ... "To edit your photo to bring it within a desired color space, click Create Proof Copy. Lightroom Classic creates a virtual copy that you can adjust to print as you desire. Then, make your adjustments." I certainly don't want to go making changes to the original that I use for other purposes such as web or social media that I then have to go back and undo. I want a "print" copy that I can reprint whenever needed. This seems to be the purpose of a proof copy. I have tried adding the original to a print job and you are correct in that it does not get added to the other jobs. However I often need to adjust sharpness, mid-tone contrast or tone to suit printing and I don't want to do that to the original. This definitely appears to be. LR bug, I have posted my original comments in the Adobe Community.
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