When a player looks at an enderman but then not away, the enderman gets “stunned”(correct).
[X] When the player gets too close, the enderman just stands there instead of teleporting.
In 1.15.2, “stunned” endermen teleport away when a player gets too close(more specifically close enough to possibly hit the enderman), but this is no longer the case in 1.16 pre-5

+Screenshot of an angry enderman standing still while 1 block away from the player.