Udon version 2020.04.30.18.46
UdonBehavior does not execute OnAnimatorIK(int layerIndex) if it is called from the uGUI Event.
This may be due to the fact that the symbol string of the compiled Udon Assembly and the symbol recognized by Udon Behaviour are different.
(1st image) Udon Assembly's Symbol
(2nd image) Symbol recognized by UdonBehaviour(UdonBehaviour.cs, line 279)
It should correctly be "_onAnimatorIK" with a capital K at the end ( https://docs.unity3d.com/ja/current/ScriptReference/MonoBehaviour.OnAnimatorIK.html ).