I'm following the Tutorial on Face Key Editing (https://www.youtube.com/watch?v=8OCkFY-R3ME)
But when I add an additional 'Muscle' keyframe between two existing Expression keyframes this produces a glitch in the animation, as if the character has a facial tick.

I've selected all the keys and set the Transition Curve Preset to Smooth, but makes no difference. It's as if there's an additional animation data I'm not seeing.
Can anyone help? Is this a bug or am I doing things incorrectly?

                                    

Unity Virtual Reality Developer