Once you have problem at field, it is hard to make the conclusion of failure source because there are so many potential factors such as gasket material, bolting material, excessive bending moment, chloride attack during hydro-test, and other metallurgy problems. My understanding is ; Austenitic stainless steel(321/347) pipe is generally used for high temperature hydrocarbon/hydrogen service in refineries.
You’d better let members get more detail information about your problem, that is, service fluid, temperature, bolt material, and gasket material, max. bending moment. Also, please let us know if you have a leakage problem in only single point or all flanges ?

I would like to point out a possible factor; if a high pressure system fluid is exposed to elevated temperature(extremely high temp), alloy or austenitic stainless bolt materials(A320) may cause flange leakage problem. In this case, Martensite stainless steel bolt(A453-660) material is a solution because its thermal expansion coefficient is much comparable to austenitic steels bolts at high temp. and high tensile load.