I just went to a Panasonic store and found that its demo 55CX700H also has the same symptom. Actually other sizes and models also demonstrate a similar symptom to different degrees, although the demo 49CX700H does not (or is much less severe.)
Over the years I have learned that there is no perfect TV - whatever brand you choose there's always going to be some flaws. There is always some tradeoff. As good as plasma was, my P50V20H had banding (which is normal and is a limitation of the technology) and green blob issue (not normal) but less serious than the green blob in ST30 series. I checked out a shop demo unit it also had a green blob issue. Shun Hing also brought another unit to my home to prove that it also had green blob. I gave up.