Pretty sure this used to work, but looks like at some point during the frequent updates to GearVR / Oculus, you now just get a flat pano image rather than stereo. My guess is that the 360 Photos app is now looking for a different image format (could be related to image resolution as the test image I used was the 8K one from earlier in the thread that Marcin made; or perhaps it now only works with cubemap rather than spherical) - best bet would be to follow up with Oculus and see what format it is expecting these days, or of course use CoronaVR :)
I haven't found any conclusive or useful info with a search for how 360 Photos works with stereo panos these days (and in fact, due to that, I've been using a Cardboard app to view stereo photos or panos if I want to check them outside of CoronaVR, as just doesn't seem to be reliable info on what 360 Photos is expecting these days for stereo images).