More testing: The level 30 went to Stonetalon and focused on killing and skinning exclusively green mobs, primarily level 23-25 Pridewing Wyverns, which per WoWhead data are at the upper bound of the level range to drop PL. Another sample of ~200 skins yielded zero Pristine Leathers. On at least four occasions I skinned a mob and was greeted with an empty loot window, which immediately closed. The body on the ground would then say “skinnable” on its tooltip but not allow me to actually skin it again. Perhaps scarce drops disappearing in to the aether? I don’t know.
Meanwhile the level 25 shaman continues to get Lichens like crazy.