This is an infuriating issue for resto druids in the open world.
If you have the item “Temporal Spellthread” on your pants, then when you enter PvP combat, start a duel, change subzones, or sometimes seemingly randomly, you will drop from full mana to 10 or 20%.
I’ve only had this happen on my resto druid (I also have a holy priest and a warlock and it does not happen).
Please fix this as it’s gamebreaking for world PvP.
Other people have also had this issue (Mana enchant causing Resto druid to drop from 273k mana to 50k in pvp combat) but that topic is a month old and it still has not been added to the list of known bugs or acknowledged by blizz.