CATS Apply Rest Pose Behavior
TL;DR CATS' Apply Rest Pose only applies to meshes parented to the armature, rather than any mesh with an Armature Deform modifier.
If you Apply Rest Pose and your meshes all jump back to their old position, this could be why. CATS does not apply the changes to any mesh using the armature via Armature Deform modifiers. The meshes need to be children of the armature.