Mine does the same when "Record video at 60 fps" is turned ON. The default is set to OFF.
I had two brand new 6 pluses with the same problem. One restored from backup and the other set up as new. I've been trying to troubleshoot with Apple support with no luck. The issue was sent to their engineers.
I hope they can fix it in the next iOS update because one of the main reasons why I upgraded to the 6+, was because of the advertised HD video. I'm surprised more people haven't stumbled upon this bug. I'm guessing lots of people haven't turned 60 fps on yet.