How to edit gain map image to preserve HDR in Live Photo

I have an app that allows you to edit your photos. To preserve HDR, I edit both the SDR image and gain map image, like so:

let sdrImage = CIImage(data: data, options: [.applyOrientationProperty: true])
let gainMapImage = CIImage(data: data, options: [.applyOrientationProperty: true, .auxiliaryHDRGainMap: true])
// edit them...
try CIContext().writeHEIFRepresentation(of: sdrImage, to: url, format: .RGBA8, colorSpace: colorSpace, options: [.hdrGainMapImage: gainMapImage])

I also support editing the still photo in Live Photos. To do this you create a PHLivePhotoEditingContext, set the frameProcessor block which gives you a CIImage that I edit when the frame.type is .photo, then you create a PHContentEditingOutput and call saveLivePhoto. I’m not seeing any way to preserve HDR here. Interestingly the frame processor is called twice with .photo frame.type, but I don’t see any difference between these images. How can I edit a gain map image to preserve HDR in the still photo of a Live Photo?