I remember some discussion on this topic a while ago, and pets were supposed to have a separate target cap than players. However, this seems to not be the case for hydra:
As you can see, of the 5 targets, 1 went to a player, and 4 went to hydras. There were definitely enough other players within range. I'm not hating on hydras, I use them too. This is a bug that should be fixed.
Comments
Epic Stronghold
Block timing explained