Natural Gas: "Strange deviations at the UNG ETF"

From FT Alphaville:

We’re not sure what to make of this, and perhaps the data is misleading. But there have been some very strange things going on in the world of NAV/closing price deviations at the United States Natural Gas (UNG) ETF of late.

Courtesy of Morningstar, an investment research provider, here is the extraordinary closing price/NAV deviation from Monday, 11th May:

Meanwhile, this was the picture on May 12th:

And this was the picture on May 13th, as per data from the UNG itself:

As Morningstar highlight in this article, the whole point of the ETF construct is that market arbitrage forces close out major deviations between market prices and NAV....MORE