Gyroscope.attitude rolls back?

Hello!

I'm trying to build a camera driven by an android phone's gyroscope. It all initially seems to work fine, but when I make a gradual turn and then stop, the camera will slowly rotate back to its starting orientation after I've stopped. This seems to only happen with yaw rotation. Pitch and roll seem to work fine.
I don't seem to find anyone talking about this issue.
Is this a problem with my phone, or is something else going on? I'm using a Nokia 5.