My iPhone 6S Plus seems to default to showing the landscape home screen if I unlock it while it's sat on a flat surface, i.e. a desk or table. The phone is definitely in portrait mode when I place it down and lock it, but when I come back seconds / minutes later to unlock, it's reverted to landscape.
Is this likely to be a software issue, or could there be something wrong with the handset? Mildly frustrating.