You may have a leak that only shows up under higher pressure. Could be anywhere there's a connecting point from the tank to the keg.
I have a dual regulator and had the same thing happen to me this week. The keg held pressure but when I connected the keg to the external gas line, the tank emptied in two days. I know I had enough gas in there, and have used that line many times in the past. I had a small leak on the post connector that only showed up under higher pressure. Lucky for me I had a full 15# tank as a warm spare.