your Power Mgt settings. Make certain that you are turning off normally rather than putting the system into standby.

Set the Pwr Mgt to "User Define" and then make certain that no Pwr Mgt functions are enabled.

In a normal system, the Master Reset line is held down by a line from the power supply [a somewhat simple logic circuit]. The Master Reset line gets realeased some where between 100 to 500 milliseconds after the Power Supply's main rails get turned on. This allows enough time for the voltages to rise from zero the normal levels. The line from the supply is known as "PWR OK". If that doesn't release, then pushing the master reset switch should not cause a master reset. If the reset button function works, it would seem that the power supply signal is working but the logic circuit on the mobo is not working properly. Possibly the Power Go signal level from the power supply is marginal and the reset button actuation makes the logic circuit start properly and release and then actuate the reset. Kinda guessing.

If that is the only problem, might not be worth fixing.

Of course you should make certain that all of the plugs and connectors are fully seated.