Like Winmagman said the only thing I can think is the coating is bringing pressure and FPS down. I shoot a 28" Broughton and your load in mine ran 2900+
With that coating I would work up and look for pressure you may be able to run higher.
I Know Tim North was using R-22 so I gave it a try and it shot so good I left it alone. My load never made it as high as his about 3 grains under but I also dont know if he uses coated bullets.
I think I know why he said to use R-22, It seem a little tough on throats. I've chased the lands over 100 thou and know the Barrel grave yard is in sight
The new barrel is sitting underr the bench waiting.
H-4350 worked well also 22 just gave the best speed and accuracy in mine I run it at 2950fps I had a 3040 FPS load that was the best but brass cost too much for that load!!
Have you ran it over a 2nd crony just to check it?