I'm still waiting for someone to definitively prove that the old timers of yore took their mash pH sample readings at mash temperature.
The following excerpt from Briggs et. al. is what we need to "trust but verify":
"The pH of mash or wort alters with the temperature. At 65˚C (149˚F) the pH of mash will be about 0.35 unit less than at 18˚C (65˚F), owing to the greater dissociation of the acidic buffer substances present. Therefore, enzymes whose pH optima are known from determination at 20˚C (68˚F) appear to have higher pH optima in the mash if this is cooled, as is usual, before pH determination. An infusion mash is best carried at pH 5.2-5.4. Consequently, the pH in the cooled wort [from this mash] will be 5.5-5.8."
https://byo.com/mr-wizard/setting-record-straight-mash-ph/