Is the evap fan icing over as well?
Maybe check if the drain tube under the evap unit is OK and not blocked, preventing the defrost meltwater from fully draining away, so that it refreezes and eventually builds up covering the fan and the evap unit.
Disconnect the power to the refrigerator and then disconnect the defrost heater from the wiring harness and check the resistance of the defrost heater with an Ohmmeter.
According to the tech sheet for the model, it should measure 550-650 Ohms.
The tech sheet wiring diagram shows a thermal fuse in the heater circuit. I realize that you said that the heater is working so the thermal fuse must be OK Looking at the parts list (see below), the thermal fuse is part of the evap fan harness cable and it is there as a safeguard to prevent the evaporator unit getting too hot during a defrost cycle.
The wiring diagram also shows an "evap sensor"?
There's no mention of an "evap sensor", thermistor or thermostat in the parts list for the Freezer compartment or Unit parts section (only a thermistor for the Refrigerator compartment) of the refrigerator, but there is a component called a "heat probe" (p.16 Unit parts, Item #35 part # W10895163 (now superseded by W11568748) which is mounted under the defrost heater so I'm wondering if that is it. Is the "heat probe" used as a temp sensor for the freezer compartment, I don't know? Looking at the image for the part in the link, it seems like it is just a metal bracket of some kind
The tech sheet linked above also shows what lights are showing on the control board in the diagnostic mode if there a problem with the defrost sensor (the tech sheet wiring colours for the "evap" sensor may help to find it.) or the mainboard+the "defrost sensor"
The defrost cycle may be a "timed event" under the control of the control board as there isn't a separate defrost timer, so you may have to time it and see if it is long enough to melt all the ice..
With most other refrigerators where the control board controls the defrost cycle time, the defrost cycle occurs either every 8-10 hours or every 8-10 hours cumulative compressor run time (which is usually longer). The actual length of the time between defrost cycles depends on the make and model.
Cumulative run time means that the control board times how long the compressor has been actually operating, as it doesn't have to be operating continually. It is stopped when the preset lower limit temps are reached and then started again when the temp rises above the preset upper limit of the temp range e.g. depending on the ambient temps, overnight it may not run that often at all because the doors are shut for longer as it isn't being accessed that often like during the day, so the cold air isn't lost so much etc.
The time duration of the defrost cycle is approx. 20-25 minutes.