From conversation with aircrew down at MPA last year there seemed to be a belief that the seal around the top of the nose was of more concern than the beard, but after a jet had to descend rapidly owing to oxygen issues a high rank in charge decided that beards were unsafe without having much scientific evidence either way so banned the fast fleet from growing them (albeit keeping the usual exemptions). Others in charge decided solidarity was necessary so rolled the same rule across slower aircraft.
How much is true I'm not sure but this tour there are fewer beards than there were last year when the rule had just changed, I miss the weekly beard parade with the CWO inspecting each beard for compliance...