Quite often Vanish is acting as though it were not pressed at all. It will not drop threat, it will not drop target, and will not drop combat. Historically it could always be resisted in terms of not dropping combat, but it never used to resist dropping threat. I am unsure if it depends on pressing another button too soon, or if the chance to resist is meant to suddenly be tied to the aggro drop, but this is definitely a “change” and it tends to be problematic when the threat drop mechanic just doesn’t work.