When enderman are on fire but not taking damage from the fire, they still teleport reguardless of the fact that they are not in danger.
Steps to reproduce:
Apply fire resistance to an enderman (either by the /effect command or by splashing it onto them)
Apply fire to an enderman (either by setting the block they are on to fire or by hitting them with fire aspect)
Note that they do not teleport constantly, or in a "crazy" manner (such as displayed in this old bug report), but rather after a few moments. This bug report is reporting the fact that they are teleporting despite reason not to, not the frequency that they are.
Since enderman are hard to track, I recommend before attempting to reproduce, mob spawning is disabled and that the enderman is placed in an area where it cannot teleport very much (such as the spawn platform on the void superflat preset).
I was unable to observe a difference in behavior from a normal enderman idly teleporting and the teleporting of one on fire with fire resistance