This was a slightly surprising find. If I take a Photo on my iPhone 11 using the built in camera or the lightroom camera. When that image is syncronised to Lightroom desktop and 'Export Original' and open in Photoshop or 'Edit in Photoshop' there is no Depth map channel.
But the depth map is in the image, for example lightroom mobile and the (Apple) Photo app can see (and edit) it. If I sync the Photo to the desktop outside Lightroom and open in Photoshop then it is there.
So export original isn't persisting everything (I presume as Lightroom desktop doesn't support Depthmap yet, or so it seems) it is unable to rewrite this data to disk.
If I add the photo and add to a collection which I set to "Store Locally" I can then navigate to the folder and select the photo -> edit in photoshop and the depthmap exists.
This would seem to be counterintuative, export original doesn't persist all the information in the cloud original, now whilst I can see how this could happend it can't be working as intended?