Paramaterized sketch quietly loses mirrors

[mark.dunn.and.sharon.king]

So I have a series of sketches in the included Fusion 360 Archive File, which I have used to home in on this problem. 

On:

Component - Pump:1

Sketch - 'Internal midpoint Mirror'

Action - Select Modify/'Change Parameters'/'User Parameters'/internal_teeth

               Change the value from 3 to 4

Expected           - All the lines drawn on the left of the centreline are mirrored on the right.

What Happens - A few of the mirrors go missing (leaving blue, unconstrained lines)

                              These unmirrored artifacts build up causing greater problems later

Detail

This is a set of sketches to create a pump:

Plan - includes some deliberately unconstrained lines such as the reference centre lines of the pump parts so I can watch them interract.

Internal Reference Locus - tracks a point on the external gear with reference to the internal gear. This is used by 

Internal Midpoint Locus - which builds one of the walls of half a tooth

Internal Midpoint Mirror - which uses symmetry to create another wall for a whole tooth

Internal Midpoint Pattern - Which repeats for the number of teeth on the internal gear

Originally these were all in the same sketch but I have split them out for debugging.

On the plan, you can grab and move either the internal centreline or the external centreline and see two teeth interracting.

It sometimes sticks, and I don't understand why.

Sometimes the behaviour changes completely with the gears turning the wrong way. I think this is because Fusion 360 converts negative angles and angles over 180 degrees into its simplest form and occasionally loses track of the real angle.

In the 'User Parameters' I have a bunch of variables such as:

    'internal_reference_radius' to control the overall size

    'internal_teeth' to choose the number of teeth on the internal gear (the number of teeth on the external gear is one less)

    'tooth_radius' to determine the size of each (circular) tooth

I have tried many ways to stop the 'Internal Midpoint Mirror' but I have run out of ideas.

Is it user error or a bug?

Please feel free to ask for further information.