Rosebud,
I've just doublechecked this, by click/dragging a corner, the image flips focus for me in both pan crop and track motion. If Size About Center isn't enabled, the image will flip to one side or the other and lose its center anchor. Is this what you're seeing?
Track motion didn't change much from V5 to V6, just presets added back in.
Rosebud, I'm not understanding what you're thinking is a bug? It's working normally for me here. Dragging a corner flips it the other direction, or right click/flip works for me as well