CAUTION: The following post includes load data generated by calculation in QuickLOAD (QL) or Gordon Reloading Tool (GRT) software is based on particular powder lots, the assumption the primer is as mild as possible, and assumptions about component, chamber and gun geometry that may not correspond well to what you have. Such data should be approached by working up from published starting loads. USE THIS DATA AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL, nor QL's author or distributor, nor GRT's author assumes any liability for any damage or injury resulting from use of this information or information derived from it.
The heavier bullet lowered the pressure? That's bizarre! I tried the two Welcdcore bullets in QL with a 51-grain charge of RL-17 and got 47,149 psi for the 220-grain and 54,031 psi for the 250-grain, so the 250 RAISED the pressure 14.6%, as expected. You did something funny there.
More importantly, I ran both QL and GRT and got the biggest disagreement between the two that I've ever seen. Using QL's case water overflow capacity and 3.228" length in both, GRT predicted a peak pressure of 66,845 psi for the 220-grain bullet load and 82,150 psi for the heavier bullet, the heavier bullet producing 37.5% more pressure.
Looking at the powder, the two programs have some disagreement about its properties, as you might imagine, with GRT giving it both a somewhat faster burn rate and higher energy content than QL, among other things. So I looked at Alliant's load numbers for 308 Win (they don't have 8×57 Mauser), which is a little smaller in capacity (QL says the case has about 63 grains of water capacity, while GRT says 59 grains, about like a Winchester 308 Win case), but not by much, and there a 50-grain load is the max for a 165-grain bullet. Comparing it to results in GRT, GRT matches very well. So I think QL is just way off with its Re17 model one, and GRT is much closer. So I think I would want to have a strain gauge and work the load out on a pressure trace for this powder. Something is askew.