Hi @mandakato
It may be a faulty defrost thermostat.
The function of the thermostat is to turn off the defrost heater during the defrost cycle so as to prevent damage occurring to the evaporator unit if it overheats, before the end of the defrost cycle.
The supplier linked above has a video that shows how to access it
Here's a video that shows how to test if the thermostat is OK or not
If it is faulty, search online for WR50X10068 to find suppliers that suit you best.
If the defrost meltwater is entering the freezer compartment. check that the drain tube located beneath the evap unit and leading to the evaporator pan underneath the compartments is not blocked.
The meltwater should drain to the pan and be evaporated naturally. If the tube is blocked for whatever reason, then the meltwater backs up and overflows back into the freezer compartment and then refreezes once the defrost cycle is over and the compressor starts operating again to get to the normal set temps until the next defrost cycle.