Currently the damage mitigation and stamina drain from the Esoteric Environment Greaves mythic turns off permanently on falling below 33% stamina and does not become enabled again until you un-equip and re-equip the greaves. Additionally, death, changing zones, changing skills, and changing attributes also causes the set to turn off, presumably because these events cause a reset of your resource pool and a temporary drop below 33% stamina.

@ZOS_GinaBruno @ZOS_Kevin