That should never occur, and is definitely NOT "bolt relaxation".
You might - very, very low probabity! - have a bad gasket. Failures happen. But I've never seen a leak fail like due to bad gasket since my hydro test in 78.
How many joints are leaking?
You almost certainly have a bad assembly <- From bad assembly workmanship.
Wrong final torque values.
No intermediate torquing being set.
Bad torque wrench, or no torque wrench at all!, or out-of-cal/broken torque wrench. (or other torquing mechanism.)
Not tightening in a star pattern.
Using a sledge hammer.
Bad alignment.
Movement of the pipe or heat exchanger; bad foundation or bad pipe support twists the flange apart when the HX is drained or filled.
Bad original fitup -> So the flange never was parallel and at the right gap.
Bad fitup: Excess gap, no gap at all (the gasket is pushing the flange apart.
Scratches or cuts in the gasket face, or across the gasket.
Dirt or rust or gunk on the faces.
UNDER OBSERVATION! Take the flange apart.
Clean it, verify alignment, verify "no movement" happens.
Verify assembly and insertion position and handling and care care of the new gasket. (DO NOT re-use a gasket that has failed. Ever.)
Verify the torque values assigned for that size flange, those bolts, that brand name of gasket.
Verify the torque wrench. Prohibit "assistance" or extension handles or breaker bars or chainfalls or pry bars or other "we have always done it that a way" claims. If the foreman persists, fire him.
Verify original starting torque is correct. The 1/3 torque, 2/3, 90% and final torques are correct, are correct ON EACH BOLT in the pattern, and are properly using the torque wrench.
Workmanship - It ain't for weenies nor macho's.