Thats not bokeh artifacts rather a fence or mesh behind the bird ...
not smooth due to OOF background structures not blending nicely
still qualifies IMO
find a busy scene at mid distance and look at it thru that lens, then change the FL and aperture and you may find a few situations where the background bokeh is heavily marred by spherical distortions and looks, well, gross.
Small tradeoff for the otherwise excellent performance of that lens.
I liked the old non-IS version for making smoother background more often but I got the v2 for the ultimate sharpness and contrast.