First, that is not a normal trajectory for pH during a mash. Generally, I have found it takes a drop with the addition of the grains and mixing thoroughly to within a few tenths of target, then it slowly equilibrates and will come down by 10 to 15 minutes into the mash to target. It may drop another tenth or two by the end of the mash, but by then the enzymes have done their thing.
I have not used acid addition to control my mash pH, preferring to use water salts and acidulated malt to get in the range I want. I will say that I have found that the BeerSmith recommendations for acid malt is about 5x to 10x higher than I usually add. I am guessing that the model Brad uses would do the same for straight acid additions.
The other issue right now (being that this is a new feature, there are some gaps in performance), is that the model does not take into consideration acid malt in the grain bill nor water salts added to the ingredient list which are not linked to a set water profile. To get the water salts to work properly, I have created a water profile for the different types of brews that I make [hop forward, malt forward, balanced, stout] and add the water to the recipe and say 'yes' to having the salts added by the program to the list of ingredients. Honestly, I just started doing this just before my Spring/Summer hiatus from brewing, so I haven't fully vetted it out well.
I think the program is set up this way to prevent the addition of salts for flavor and not in the mashing from affecting the mash pH calculation.