Hi Fran,
I re-read your first message, so are saying the issue only occurs when the device is horizontal, versus in vertical orientation?
The Survey123 field app only supports a vertical orientation, it is not designed to work in horizontal orientation on smaller screens, only on tablets will it go into true horizontal mode. I believe this is the likely the cause of the issue, as when you turn the phone horizontal, the app is locked to a vertical orientation, so you will see that the menu bar and other parts of the application do not flip to the long edge. This is likely the cause of the orientation difference you are seeing. Whilst the photo can still be captured with the device in horizontal orientation, the app still thinks it is in a vertical orientation, and therefore the reported orientation will match that.
Regards,
Phil.