OK.

It's that maximum combination that's causing the commotion here, that is, your "L50 = max(L34 through L49)".

For this combination, we select the maximum value from your list of load cases (L34 ... L49) for every reported response (such as X,Y,Z displacements, forces, and moments, and stress). Unfortunately, pressure is not a reported response in these MAX/MIN combination cases. Since we have no pressure carried over to this report, there is no line pressure for this flange report. Does this make sense? (I am not asking you if our implementation makes sense; I'm asking if my explanation answers your question.)

I apologize. We will have to fix something here.

In the meantime, I suggest you avoid flange checks on MAX/MIN combinations. Pressures are carried forward for other combination types since the resulting pressure is used to recalculate stress. But I am skeptical of the value of these other combination types. For example, if I have P1 in LC5 and P2 in LC7 and if I look at LC7-LC5 as an algebraic combination, the line pressure will be (P2-P1). I don't think we should be evaluating pressure differentials. To avoid this, be sure pressures are only included in only one of those load cases you are combining.