Its probably always been that way ...in hollow trees, with solid floors and now with mesh floors ... I suspect that there has been a whole host of pollen mites that survive on the debris at the bottom of a hive .. there's always a bit of pollen on my inspection boards in the vicinity of the frames they are working - it's one of the ways that I can tell what's happening inside the hive.
I don't know whether bees cleaned up this pollen when they were in hollow trees and on solid floors but obviously with mesh floors it's not so easy for them ... indeed, leaving a bit of pollen in the bottom of a hollow tree might have contributed to the eco-structure of other insects that clean up the bottom of the hive, Bees rarely do much without reason ...?
I have noticed on the large landing boards of my poly hives that they fastidiously clean up every scrap of pollen dropped by the bees landing with full pollen sacs ... indeed they seem to hoover up pollen that I can't see as well ...
