Piping is frequently designed such that it its MAWP is limited by the B16.5 flange rating for the class of flanges used. Though this is frequently done, there is no requirement that flanges be the limiting component of the MAWP- ANY component may limit.
It is required that this piping be tested to 150% of the relief device set pressure, which may be set as high as the MAWP. That means of course that for piping designed for low temperatures and with MAWP limited by flanges, the flanges will experience 150% of their rated maximum pressure during the hydrotest.
It is permissible to reduce the calculated hydrotest pressure per the applicable design code to avoid putting components into yield (due to big stress ratios between the test and service temperatures for instance). But there is no requirement to limit hydrotest pressure to no more than 100% of the B16.5 flange rating!
This relief device is allowed to "accumulate" as much as 110% of the relief set pressure at its full rated capacity for typical relief events, and up to 121% for fire relief events. But since this is less pressure than the piping experienced during the hydrotest, this should not be of design concern.
Your corporate standard is therefore spewing BS in this particular regard. As owner though, your firm can feel free to assign whatever conservatism beyond code in design that they are willing to pay for.
Normally if a flanged joint leaks under hydrotest at 150% of its rated pressure, that would be considered a failure of the test. With new gaskets, flange faces and bolting, B16.5 flanged joints shouldn't leak even at 150% of their rated pressure. Leaks at the body flanges or packings on valves? That's another matter entirely!