Enable/Disable Face Tracking Problem

I’m working on a narrative project that shows normal 3d, then AR, then pure UI in a sequence.
I need to enable Face Tracking for a while, then disable it and activate other cameras for more basic rendering.
I can’t do this by switching scenes currently. When I try to start a scene with the MARS Session/camera disabled the camera feed doesn’t properly composite in the background on my Android build tests when it is activated a minute later; it just goes black (though I see the test footage correctly in editor). It works fine if the MARS Session is enabled at the scene launch.
Is there a recommended way to achieve this that will be more stable on Android (and possibly iOS).
I’m using Unity 2020.3.14 with AR Foundation 4.17 and MARS 1.4.1

If you need the external camera feed but no the proxies can you enable the MARSSession from the start and only enable the proxies when you want them?

An alternative approach is to set the scene up as a regular AR Foundation scene and then enable the MARSSession when you want - it will reuse any existing AR Foundation components it finds in the scene.