This seems to occur in any resource pack that uses JEM on ravagers, as I created a new one using the base ravager Blockbench model and the issue occurred even in that state.
In vanilla Minecraft, the ravager's lower jaw should rotate with the head.
^^ Image of jaw issue using base Blockbench JEM for ravager.
^^ Normal ravager head rotation. (Jaw has also moved.)
Ravager jaw does not pivot correctly.
This seems to occur in any resource pack that uses JEM on ravagers, as I created a new one using the base ravager Blockbench model and the issue occurred even in that state.
In vanilla Minecraft, the ravager's lower jaw should rotate with the head.
^^ Image of jaw issue using base Blockbench JEM for ravager. ^^ Normal ravager head rotation. (Jaw has also moved.)