I’ve been using the animation rigging package for quite a while but I’ve never struggled with this issue. When I use the two bone constraint, the arm will weirdly twist. I’ve tried many things like redoing the rig on the model, making it humanoid instead of generic. But none of them work. I tried using a free IK asset to see if it was an issue with my animation or rig, but it actually worked fine. So, why does this happen when using the animation rigging package?
I didn’t understand the problem well but I think I can give a usefulish advice:
(it seems that) the position where you start the preview or animation affects the behavior of ik in unity,
to get my ik to work properly, i always bend the armature slightly the way i want before activating the ik components,
such as bending the knees or arm slightly,
not doing so sometimes results in strange movement of the root bone.