Freezer intermittently melts ice then refreezes

My 12 year old Amana ASD2522WRB03 side by side freezer has an intermittent issue where the ice melts then freezes again into a “block”. This was happening every few months, but is now happening more often-every few days. I assume the defrost cycle is staying on too long or coming on too often. The coils under the unit are free of dust/debris, the compressor and fan both are running. The door seals are all tight. Inside the freezer, the evaporator fan runs, the evaporator is not frozen up, the heater seems to work when in the defrost cycle. A friend suggested the defrost timer, but my unit does not have this “mechanical” device. I replaced the electronic defrost control board, but it did not fix the issue. I was going to try the defrost bimetal, but could that be intermittently faulty or is it the type of component that would not work at all?