Separate from the fixed MC-71256, as this isn't about them freezing in place.

When you look at an Enderman and they freeze, they will not immediately relocate their head position to be facing the player that triggered the hostility, nor will they track their movement. They will instead be stuck facing wherever they were last looking. If the player is close enough they may passively glance at them, but this is separate idle behavior.

I'm unable to provide my own footage as proof other than memory of it happening, but I was able to find two videos demonstrating this old behavior online.

1. https://youtu.be/0skRdWJMPPA?t=2 - At 0:02

2. https://youtu.be/-nkyzCp_RQk?t=54 - At 0:54