CCD flats show regions of vertical structure from top of detector

There are regions showing an "icicle" structure seen at the top of the flats for (at least) one detector, where the gain is slightly lower than elsewhere.

So far this has only appeared on the left r1 detector.

The attached images are from a superflat built from frames 7260-7264 on 2015-12-22. For one of the amps, the gain above the edge is 0.996 vs. 1.004 below.

[ File attachment is not working, so here is a link instead. ]

https://drive.google.com/drive/folders/0Bzyik_ZyeKPfNy1aMVhBbU9fSGc?usp=sharing