When Hex uses Jinx on mobs gargantuans will not attack those mobs until the effect (i.e. being a pig) wears off.
Sometimes fetishes will also not attack until the effect wears off.
Follower/pet ai seems to be that it will react to threat around your proximity.
I noticed this with my new necro, and I just tested it with HT garg WD (non-season, but doubt it will make a difference). It will always attack hexed mobs if pets were attacking them before they were hexed. They will sometimes not attack when I hex a group of mob before my pets engage. Sometimes they will attack (probably because there were patch of mobs that weren’t hexed).
So, not really a bug, but more a game mechanic. Let your pets or mobs engage first before you hex them.
You can’t let you pets attack before casting Hex when he’s already out.
Even if it’s a game mechanic it needs fixed imo. WD’s don’t need a damage nerf.