Local Multiplayer - Oculus drops view while Vive works fine

We have built a basic VR app with Blueprints. It allows a patient to view a skeleton in VR and a practitioner to view the mirror on desktop. Furthermore the practitioner has a control panel that can manipulate the skeleton whilst the patient views the results.

It works by running two versions of the game in local multiplayer. First the practitioner loads up as Player 0 (not in VR) the second player is always the patient and is in VR. The Vive works absolutely fine, very nicely. If you use the same build with an Oculus, if you click the mouse anywhere outside of the HMD mirror window, Oculus HMD goes black. Fiddle with the console and then click back in window and it instantly updates.

This appears (to me) to be a quirk of Oculus not being used in a full screen game? But I have no way of proving it. I am hoping that someone has experienced and discovered a work around for this problem. Oculus App settings are so minimal it would have to be a default.ini or engine override?

Many thanks, Sam,