A mob was just chasing me as I ran but I decided to stop and fight it, but after we fought for a second it just disappeared and reappeared back at the spot where I first encountered it. I’m sure by the time we actually started fighting it had reached its leash location, but instead of the usual run away and heal move they do, it just vanished.
The same thing happened in SE p3 about a few hours ago. After the bridge my group accidentally aggroed the patrolling destroyer crabs. They chased us up the ramp before the boss fight so we just decided to dispatch them. At about 50% health, all three disappeared.