3D Camera is glitching out of focus while moving camera and changing focus distance accordingly

Is there a reason you are using such a long telephoto lens for the camera? Your angle of view appears to be about 15.9º. That would be about the same as a 2000 mm lens on a standard movie camera.  That angle of view and a zoom value of 13750 tells me that the comp must be 4K. The camera and the target layer (Layer 11) are also offset to the left of the comp center by almost twice the comp width.

The long lens and the distance from the comp center could be causing problems. If this were my project, I would put a 3D null in the same position as Layer 11, then parent all layers, including the camera, to the null, then reset the position of the 3D null to comp center to get things lined up with the compositions 3D world center. Just doing that may solve your focus shift problem.

I would also consider using a much shorter lens, like a 50 or 80mm lens, spacing the other layers farther apart, and using a wider aperture for a shallow depth of field. Only about 100 pixels are separating the layers. That means the aperture would have to be wider, but the results would also be better, and you would not be dealing with values that could possibly throw off the calculations.

You are also working with vector layers, so I suggest that you turn on Collapse Transformations. You should get a better-looking depth of field effect.

I would also use an expression to tie the Focus distance to the distance between the camera and the target layer. If you named the Layer 11 "Target Layer," the expression would look like this:

point1 = position;
point2 = thisComp.layer("Target Layer").position;
length(point1, point2)

That expression will keep the center of focus precisely on the anchor point of the target layer no matter where you put the camera. 

When I set up a shallow depth of field and need to change the focus from one layer to another, I use a null for the Target and then animate the null position using the position of each layer I need to bring into focus for the keyframe values. Tying focus to the null also gives you something you can see in the different comp views that tells you exactly where the center of focus is.

Let us know if any of these suggestions help.

A pin hole camera is based on a cosine function, so the interpolation is never perfectly linear. You need to tweak the value graphs accordingly to mimic this behavior.

Mylenium