Can you believe it? UNG same in Jan. expiry as this expiry.
Here is a bit of trivia from CME. It seems there is something in Jan to April, that supports U on expiry in general: 
Trading at Settlement (TAS) Trading at settlement is available for spot (except on the last trading day), 2nd, 3rd and 4th months and subject to the existing TAS rules. Trading in all TAS products will cease daily at 2:30 PM Eastern Time. The TAS products will trade off of a "Base Price" of 0 to create a differential (plus or minus 10 ticks) versus settlement in the underlying product on a 1 to 1 basis. A trade done at the Base Price of 0 will correspond to a "traditional" TAS trade which will clear exactly at the final settlement price of the day.