The Weta's follow range by default is 8. But the Avaton's aggro range is 16, like most mobs.
This means that in many cases, the Avaton summons its wetas, which just fall to the ground and do nothing, instead of targeting and pathing towards the Avaton's target.

I think either the Weta's follow range should be set to 16 by default to match the Avaton, or you could have only wetas spawned by the avaton have an NBT modifier that adjusts their follow range to a higher value.

fixed in latest hotfix