I guess any sort of trouble with the iPhone is an extremely minor thing in the grand scheme of things.
What to do about it? I can't presume to know what they tested and rejected, but screen orientation control needs to be independent of screen orientation.
It should always be at the "bottom" of the screen. If the "bottom" doesn't match whatever the bottom of your app is, that's likely because the app is configure to only appear in landscape or portrait so it only responds to a large enough position change to trigger a 180-degree flip. Basically, this isn't as simple as it appears because of the potential interactions between app types and what the OS supports.
TL;DR: If you have a landscape only app, trigger the lock before you launch the app and it should stay in place.
The iOS 9 beta had an option in Settings to change the side switch's functionality between silence and orientation lock. Unfortunately, this was pulled before stable release.