Hi I've attached a file with Victor's rig.
I'm sorry for not simplifying this, but it is a bug that I have known of for a very long time but was never able to recreate. Now I finally have an action where it pops up.
Just open the file and press shift+E (breakdowner), move the mouse a bit to modify the pose, and you'll most likely notice that the custom property that the bone has gets modified by the breakdowner. The bad thing is that this property drives the Rotation Order, so this issue can cause huge animation problems.
Just to make this clear, you'll notice that in the action the property is keyed and changes its values, well, those changes happened randomly, it wasn't Beorn who modified those values manually.
So, the thing is that the breakdowner makes values of bone custom properties jump "randomly".