You need to check your swingweight scale or something, but if you had a 50 gram on the clubs and all you did was put a 65 gram grip on them, with no length change or shaft change or anything else, it is impossible for the swingweight to have gone up to D2. It should have gone down by approximately 3 swingweight points. 4 grams of weight change on the grip end = 1 swingweight point change. 2 grams on the head end = 1 swingweight change. It has been that way since the 14 inch fulcrum swing weight gauge became the standard. The only other thing it could be is if the midsize don’t actually weigh 65 grams.
As far as not hitting them well, it could be several things. It could be the feel difference created by the different weight grip. It could be the grip size difference. I assume you got properly fit for grip size and the midsize is correct? Although there is a technical “proper” grip size for everyone, based on the finger tip to palm positioning, the actual proper grip size is the one that allows you to grip the club with the proper grip pressure which should allow you to swing without tension in your hands.
Shouldn’t be any conflicting info about the effect of grip weight on swingweight. More grip weight added will reduce the swingweight.