So, what is the pH in the mashtun at a higher temp? And is the pH I'm concerned about the one in the mashtun or the one I measure at room temp?
The mash pH at mash temp is relatively consistent at about 0.3 units lower due to the increased disassociation activity due to the increased heat content of the hotter wort. Since that is a relatively constant pH offset, we can ignore the pH value at mash temperature and do all of our correlations with wort and beer flavor and texture based on room-temperature pH readings.
Many of us know that 5.2 is a magic number in mashing. That pH value would be appropriate if measured in mash temperature wort, but too low if it was measured in room-temperature wort. You should note that now you may see ideal mash pH quoted as being 5.3 to 5.5 at room-temperature. This is similar to the 5.2 mash-temperature pH that has long been targeted, when you figure in that pH offset.
Don't worry about what the mash-temperature pH is. Just understand that if you measure pH at room-temperature, pH will actually will be lower in the mash and you don't care what that pH is since you have a safe room-temperature pH allegory to guide you.