Recently I attempted a +11 Mythic keystone of The MOTHERLODE!!, on the final boss; Mogul Razdunk, during the intermission phase where the Venture Co. Skyscorchers come out one of the Skyscorchers targeted our fury warrior and then used Jump Jet. (Jumps to a location withing 15 yards of a player.) The add jumped right out of the boss room near the entrance to the platform and reset the boss.
Its a large boss room, so I guess to avoid this bug you just need to avoid the entrance side of the platform by 16 yards, else you will have the boss reset on you and your +11 key deplete.