I've solved my own question.
After creating my test, I applied the Audience in the 'Targeting' section. But I'd also set up an 'Activity-Only Audience'/'Audience refinement' on the 'Experiences' section. It seems this one overrides the Audience set in Targeting,
Hope that helps - please someone correct me if I've misunderstood. I'm no longer seeing the activity on any mobile device