Nightmare Werepig can't be exhausted after he is freed until he gets damage.

It could be an intended behavior but I couldn't find any information about this.

When Nightmare Werepig is freed and Shadelings are killed, it becomes hostile and starts to stalk. And in this state you cannot make him exhausted from dodging his lunge attack. Notice that he changes the behavior after getting hit once. (he starts to actively run and tries to charge)

This can be a problem; you have to hit at least once without letting him exhausted, which has risk of getting hit if you only have melee weapons or have to bring ranged weapons only to trigger him once. And it's an inconsistent behavior as the main gimmick of this fight is making him exhausted and this is the only state that you can't make him exhausted by dodging his lunge attack. (He can be exhausted even in the phase 2, which he mainly threats with his groundpound attack)

I don't think it is some 'not hostile until you hit' kind of thing because the fight starts and Nightmare Werepig goes hostile right after you kill all the Shadelings. (The boss music starts)