I’m very far and late into the finishing the rigging process. Doing correctives and I didn’t notice this issue. Doesn’t happen when I rotate the FK. Is there anyway to fix this.
I’m not sure I have the guides anymore since it was months ago. 
can you send me something to check? removing models and parts of the rig
the problem was that the blade was not set correctly in the build and the to twist references where in a different starting position creating a 90 degrees flip
to workaround this without build again you need align the interpolate matrix reference with the neck IK ctl orientation
later take all the position references
reset the translation and set a value in x axis
The latest will be to update the parent matrix offset one by one
That should keep the joints in the original position and fix the flip at 90 degrees.
Note that a flip at 180 degrees still exist
Here is the fixed sample Dropbox - neck_flip_fix.ma - Simplify your life
This is one of the reasons why using Data-centric rigging is important. The ability to rebuild and fix issues is crucial 
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Ty Miquel!!! Sorry about the confusion. I am completely new to rigging this way. It’s taking me a bit to get used to thinking about data centric style. I’m very grateful that also shows me a lot about how the neck works as well.
Off topic but related, how do these work under the hood, is there a ribbon driving the IK spline or is it a custom mGear node driving things?
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is a path constrain with upvector
Path constrain? Is that specific to mGear or just maya nodes? I haven’t heard of that before specifically.
Standard Maya node.
Ah okay I wasn’t sure, I always heard it called motionpathNode
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