Your mother culture is just fine. You strips are almost certainly reading incorrectly. If it wasn't gelled at 90-90 F, then the pH was certainly over 5.0. A few hours later, it's not going to suddenly dive down before 4.5. And even if it does, it doesn't really matter.
But why are you making a gallon of mother culture? That's enough to make something like 100 lbs of cheese.