Nevermind, fixed it!
Just change the config of the existing Layered blend per bone node in the Face_AnimBP blueprint (better create a copy of it) with the two values of neck_01 and neck_02. Set the Body Animation Mode to Custom Mode and Face Animation Mode to Use Animation Blueprint and set the modified blueprint on your Metahuman, this should work.
Here’s how the modified blueprint should look like together with the config of the Layered blend per bone node in the panel on the right.
