I only play my lancer as part of a duo. In 25 levels of this, the stamina reset has happened to both of us at the same time each and every occurrence. Neither one of us can figure out why it happens at the same time, but I doubt it has to do with the mentioned suggestions other than "it's a bug."
I know that changing class, weapon, and the like will reset it as well, but I highly doubt that both of us have done it the handful of times our stamina resets. I do, however, recall it resetting most frequently at Camp Horizon.