I believe client/channel.go has a typo on line 294 that may be causing the subject issue. There doesn't appear to be a way to pass an OriginateRequest to StageExternalMedia, so I went looking and found this typo.
I think this patch will correct it. I'm trying to test the change locally, but am having issues getting the repo overridden fully for some reason
typo_in_client_channel_go_StageExternalMedia.patch.txt