Dave - thanks for the report.
Now, I kinda feel like this is a stupid question, but since you are stymied about the POI from 25 to 50 yds (as I am), I'll ask it anyway.
Did you do anything different concerning resting the rifle when you moved out to 50? I know that changing from a soft rest to a hard rest can make the barrel vibrate differently and cause a change in impact.
That's the only thing I thought of as a possibility.

Not that I could think of. I did have to raise the front part of the rest a small amount to get on target, but other then that, I have "spot" I must touch, before I shoot. I have my elbows on the table in certain spots, the side of the rest against the inside of my right forearm, and all of them were the same.

Yup - I figgered as much with as much shooting as you do, but just had to ask.
Maybe you'll just have to write it off as a fluke. Looks like you've got it pretty well dialed in now.