Perhaps someone will explain why more powder results in reduced muzzle velocity rather than diminishing returns?
That's the question I would like to know that answer to! They certainly were not compressed loads. I did a lot of testing with charges in the 48-48.5 grain range and never experienced any big variances in velocity like this.
Once the wind dies down up here I am going to test that same 50.5g and 51.0g again and see if I come up with the same results.
barrel temp, ambient temp, ammo temp, powder temp sensitivity
shooting position and inconsistent hold from shoulder fired weapon
brass volume consistency
If you can't come up with anything logical, then see if you get repeatable results by alternating shots from each load. If after 30 samples each, the mean of one load is 2x SD apart, then you can have 95% confidence that the MV going forward will be about that much different.
Either way... how did the groups compare? How did POI compare?
Richard has given you a few possibles. I would check them and anything else that may have missed, or that didn't stay constant. Anything as simple as a low battery in the chrony, chrony picking up a muzzle blast or shooting your reloads out of order. There was atleast one thing that was not working right or not constant in your reloads, chrony or methods.
Rule out as many things as possible (read "all) and reshoot to obtain correct data.