The bug in this report is almost identical to the bug in the "Left-to-Right Liquid Flow Heat Transfer Bug." The discussion about this bug can be found here.
The average value formula is grabbing the wrong mass when updating gas flows in some configurations (I haven't tested all configurations under which the bug fires, but I'm going to guess that it has to do with the order in which gas computations occur). Here is a simple example of the bug. The left side is before, and the right side
Branch: Preview Branch