If the PH drops too low, you may extract undesirable shit (tannins) from the grain.
Tannin extraction is more of a concern with high pH (alkaline) mashes/sparge.
Ah, got it. Does the PH drop because of the acidity of the grain? Or is it a byproduct of enzyme activity?
Too high or too low of a pH will result in slowing or inactivating the enzymes you need.
Other than for PH, are there any other compelling reasons to adjust water for a mini-mash?