The flange connection consists of two flanges, bolts, and a gasket. We know the dimensions of the gasket, and we're specifying the bolt tension. Therefore, we can estimate the pre-load stress in the gasket. If we add compression on top of that, we could potentially deform the gasket permanently. Now, when we slack off on the flange compression, we can introduce a leak point.

If you converted bending moment to compression on one end and tension on the other, add the compression from the bolts, and then add straight compression, and then you exceed yield, you will therefore have a permanently deformed gasket that may allow leaks on the first try.

If you exceed the gaskets allowable stress, then you'll potentially see leakage at some point in the 20 years following.