Halo on the TC 2x III

I have noticed a halo in high contrast shots w/ the TC 2x III. It is very small but it is visible.

Doesn't seem to be there w/o the TC, but it is present on the one I just purchased and the one that JRW loaned me a few weeks back.

Am I not looking closely enough on my 70-200 or is this a common issue?

I've had good luck getting rid of obvious CA by eliminating saturation of the color purple in PSE 10.

Dave