Just for what it's worth (2 cents...lol), that last TWO iPhone 6 Pluses I've had (typing on second now), have had serious freaky issues for which there have been no fix, I've had to go to the Apple Store. My reason for agreeing to go there is that none of the IOS 8 updates, backups, & restores, have not worked and the solution was to replace the phone. I'd, if I were you, just get to a store because one of the freaky issues was with the lock screen with no time like a normal lock screen. Good luck to you and I hope this problem for you is just one that can be fixed. I hate having to reset as a new phone, evidently this lock screen issue is not the only one and the issues just keep getting worse. (This is not a professional comment.) I can just empathize with the freaky issue you're having because of the many that I had.
Have a good one!!
...sent with my iPhone 6 Plus,
Kristin aka PrincessKristn on iMore