I've had this problem for quite a while now (at least since iOS 8) and with different iPhones (5,5s,6s). I usually have screen rotation lock ON since I like to use my phone while on the couch or in bed and don't want to worry that the screen rotates every other minute.
The following bug (?!) annoys my girlfriend and me a lot:
Screen rotation lock ON, I lie in bed (would be horizontal mode but it's vertical since it's locked) and I start a FaceTime call.
--> FaceTime uses horizontal mode and ignores the screen rotation lock set in vertical position
Screen rotation lock ON, I start the FaceTime call in vertical mode --> all good.
Until I switch to another app while holding the iPhone horizontally. The other app is in vertical mode (as it should be since screen rotation lock is on) but as soon as I go back to FaceTime, FaceTime ignores the screen rotation lock and switches to horizontal mode.
Solution in both cases:
1) I have to deactivate screen rotation lock (SRL),
2) hold the iPhone in vertical position,
3) wait until FaceTime switches to vertical mode,
4) activate SRL again,
5) lie back down with my iPhone in horizontal mode.
I don't see how this can be a feature! Are we the only ones who are annoyed by that?!
If you have a solution, please tell me. If that's not a bug but a feature, please describe a situation where this behavior makes sense.
Thank you a lot.