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.