From a European perspective, and ignoring Premium Economy initially, two things have been going on. Airlines like BA have been adding higher biz class ratio seating as the economy has recovered in their long haul fleets. But shorthaul, the likes of FR, EZ, 4U etc have continued to grow, with no biz class available, and the premium airlines are tending to have smaller biz cabins to reflect that. This would be entirely consistent with the number of emplanements in Biz being a lower percentage, but the number of miles flown showing a lower, or even opposite, effect.

As to Premium Economy, I'm not sure there's any sensible way of measuring that, since even now it means such a different thing to different airlines.