How to activate/deactivate a volume component inside script?

I searched thought all the forums and the answer to my question is to do the following :

DepthOfField dof;
GameManager.Instance.PostProcess.sharedProfile.TryGet<DepthOfField>(out dof);
if (dof != null)
    dof.active = true;

(GameManager.Instance.PostProcess is the Volume component, linked from the scene)

But this does nothing inside my project.
What did I miss ? Is this a bug from Unity 6 ? (my project is in 6000.0.15, also tested in 6000.0.34)