My maternal grandparents passed away on a different tithi — should I still do Pratipada Shradh for them?

Yes, absolutely. The Dharmasindhu makes clear that Pratipada Shradh is the designated day for maternal grandparents regardless of their actual death tithi. You should also perform Shradh on their specific tithi day within Pitrupaksha. Performing on both days provides the fullest coverage.