My observations are the opposite, I always check the boil pH at the end and at room temperature (60*)and my pH is always higher. It is only after fermentation that I see a drop in pH, again at 60*
When this is seen, in my experience it is directly related to having taken the "Mash pH" reading too soon within the mash, and thereby having achieved a false low pH reading for the mash.
Another way to achieve a false low pH reading it to stir the sample with the pH probe while taking the reading, thereby inducing what is called "stirring error".
And another way to get a false low pH reading is to read the pH as soon as the meters reading "stability" flag pops up. The probe needs to sit undisturbed in the Wort for a couple minutes to achieve a stable and more correct reading. The stability indicators are generally useless, but they use this feature to sell meters, so...
And yet another way to get a false low pH is to falsely believe/presume that ATC compensates for temperature, and thereby read the sample at a temperature above 68 degrees.
All of this combines to indicate that false low pH readings are vastly more commonly achieved than for false high pH readings.
But the biggest mash pH reading error of all is one related to "Confirmation Bias". This bias generally manifests itself in a totally misplaced blind faith in software prediction, such that actual pH readings are bent and twisted and contorted by whatever means that work (hint, some ways are in the list above) to fleetingly give a meter reading somewhat in line with (or better yet, spot on with) software projection. And then write this reading down because readings conforming with software prediction make one feel good. This sort of pH reading (as deeply in error as it is) is very common. But, hey, it's all about feeling good about your pH reading, not to mention impressing those who will also nod as they gullibly eat it up. right?
And I just thought of one more, which is the "I need to get in line and conform" pH reading bias. This bias has degrees to which one can be infected by it, but the worst degree of all goes something like: I've read in so many places and over such a long time period that software X is always just about spot on all the time in its prediction of mash pH, but no matter how hard I try I just can't seem to get my own pH readings to rise to this level of conforming perfection to what obviously must be some superior level software, so rather than ever embarrass myself publicly, and in order to feel like I "belong", I'll just go with the software, and better yet, I won't even bother to read mash pH's again, but rather I'll just keep repeating the mantra that software X is always spot on. Again, its all about feeling good, and this time its also about feeling welcomed as part of the group.