Arganto

If milk spotting is caused by impurities ( salts ) in blanks developing, exacerbated by sudden variations in heat and humidity ( flight to Florida from the UK as mentioned ), then proof coins using the same supplier(s) of blanks will also be prone to milk spotting. Spots on expensive premium proofs would destroy totally their appeal and probably value, unlike regular bullion, so investing in proofs is by inference high risk. Something the RM needs to take into account.
Historic discussion amongst fellow stackers about milk spotting and how it was particularly bad from the Royal Canadian Mint, seemed to suggest this was caused by chemicals e.g. borax that were used in cleaning and rinsing the blanks. If the final water rinse wasn't properly purified ( sharing the bath water for too long as an example ) or adequate, "soap suds", chemical residue ( perhaps a type of "salt" ) left behind and dried on the planchets was then baked on to the surface from the heat of pressing the dies.
I don't believe the "salt" is from impurities in the silver but more likely a "salt" present on the surface due to inadequate rinsing, or cleaning, or using mildly contaminated water in the final rinse.
That would explain the batch problem and why some but not all coins are stained or spotted as they leave the mint.