When replacing the set of t-stats the tank contact area needs to be clean AND the t-stats need to be tight against tank.
Now if above has been done and t-stats are failing......then excessive AC ripple could be the problem.
AC ripple is result of converting AC to DC...how much depends on diodes/capacitors.
To measure....test DC supply voltage at water heater. Then swap meter from DC to AC and measure......anything more than 1 volt AC could be why you are going thru t-stats.
That means converter is not filtering the AC ripple out.