Once you get your PH in the ZONE it should not change. However, different grapes act different. I know, this is one of those, Oh great, responses! All brews need a bit of acidy to give that body and flavor that would otherwise leave a very bland juice.
If I make a wine with high acid fruits i will test the ph before adding yeast and when I rack to the secondary and remove the fruit. I also quit using acid blends in my Meads as they would stick in a couple months and then I would need to adjust the PH and repitch. So just test after the fruit is removed.