Stuck in landscape mode

Hey!

I have problem with my iphone 6s, ios v.11.2.5

Sometimes my screen gets stuck in landscape mode, while the phone is locked. So I can only see half of my lockscreen and half of the password numbers, hence I can’t unlock my phone. If I start the calculator while the phone is stuck in landscape mode I’m able to unlock it and the screen is back to normal, but the symbols for battery, the clock and wifi is still stuck in landscape mode. I have tried to do a hard reset, turn off the phone, turning the rotation on and off, but after some time the problem returns. The phone rotate as normal while I use it, but sometimes it just get stuck in landscape when it’s locked. What can I do?

/ Emelie

Ps. I’m from Sweden, so don’t be mean about my english. Ds.