If you do an ASME VIII Div 1 or 2 flange 'stress analysis' for most 150# and 300# rated flanges, at the max pressure allowed by B16.5 and using a spiral wound gasket, the design will fail the 'bolt-up' but pass the 'operating' case.

Clearly there are countless such flanges out there, not leaking, just doing their job properly. The basic equations that generate the 'bolt-up' case loads are too conservative in my view.