In 5.0, the mobile depth of field settings were correctly reflected when running standalone. However, in 5.1, the DOF doesn't appear to be working correctly and the focus is off, causing a blurry image. This issue only occurs in standalone mode and not in mobile preview mode.
refer to attached sample projects and test result screenshots
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-177852 in the post.