The bugfix to leash behavior of groups of enemies makes the game behave like it did in original WoW. We discovered the problem because of Solenor the Slayer, but it was a bug with all linked combat groups’ leashing behavior. The fix only affects linked combat groups with a leash timer, so there are still plenty of situations when you might attract the attention of multiple enemies and be able to split one off.
We investigated King Gordok and Cho’Rush the Observer, and this change does not affect their behavior. During that additional investigation, we discovered a bug with Eyes of the Beast relating to threat that might explain the odd behaviors you’re describing. We’re working on a fix for it.