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It seems to be happening to me much more often lately.
The first week, I don't think I saw it once.
Now, it happens 4 or 5 times a day.
Very frustrating.

It would be nice if they allowed you to respring the desktop (kill and reload) without rebooting the phone.
At least that would be a work around.
 
Has happened to me as well with my 6 plus. I would have to reboot most of the time to get it out of landscape mode. If I didn't want to reboot I would just toggle the Standard vs. Zoom in the Display Mode settings.

I am assuming it's software related to the Standard Display mode. Recently, I decided to just use the Zoom Display mode all the time and it never happens. The home screen does not go to landscape in Zoomed mode, but all other apps which support landscape (including mail, safari etc) work fine with no orientation sticking.

Hoping Apple fixes in iOS update.
 
Software bug. Mine either gets stuck in landscape mode or portrait mode.

Only workaround is to do a reboot.

My phone was setup as a new phone so i believe it is not a problem with my previous backups whatsoever.

Even in apps and games the orientation is buggy.

Sometimes it works perfectly. Like when i am already in landscape mode, launching Safari should already be in landscape. But it is not

Someone mentioned a great work around that doesn't involve rebooting.
Change your view (Zoom or Standard) and it will "respring" the home screen.
It messed up my wallpaper setting but it is better then rebooting to me.
 
You must check the software related with it. Update your phone with a latest version of software. Also check all the parameters related with your accelerometer in the software of your phone.
 
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