Layered MetaHuman facerig 'muting' the animation beneath it

I have a MH with an animation generated using LIveLink. And I need to tweek the eyes a bit, so I’ve added a layered ‘simple’ facerig. but when i do that it ‘mutes’ the animation, particularly the mouth region. You can see the data making the lips twitch slightly - it;s like someone trying to talk but keeping their mouth closed.

Is this a bug, or am I doing something hilariously dumb?