Ice build up on rear of refrigerator near evaporator intake

Problem

I have a large chunk of ice forming on the rear panel of my refrigerator. It is near the evaporator, but not on the evaporator panel itself.

I used a FLIR to see what's going on.

The dark spot is -10ºC, and it seems to be the tube feeding cold refrigerant to the evaporator up from the condenser.

Things I've tried

Cleaning condenser coils and filters

They were actually not too bad. Condenser fan is working fine

Defrosting

I have tried doing forced defrost cycles. No change apart from going slightly crazy from the beeping. I have run about 5 cycles, and unplugged the fridge for a couple hours.

This got the spot up above freezing, but I suspect that the tube behind it is still very cold.

I also tried using a heat gun for about a minute on the plastic in front of the cold spot. This was not very effective.

Defrosting evaporator

I popped the evaporator cover panel off and unplugged it. The evaporator radiator was actually not bad. It had only a little bit of ice on it, and no ice in the drain pan. The evaporator fan itself is clean, free of obstruction, and moves freely.

Replacing some of the weather-stripping/seal around the evaporator cover panel

Some of it looked like it was not in great shape, so I replaced it with some new foam weather stripping.

I did not replace all of it, but only the part on the right side of the panel, nearest to the cold spot.

Not sure if I should bother doing the rest. It looks fine, and it didn't seem to make any difference.

Tested evaporator fan

I used a magnet on the front of the fridge to simulate closing the door. The evaporator fan kicks in pretty quickly, and sounds normal. I feel cold air coming from the vents in the front.

Isolating the cold spot from potential sources of air and moisture

I went a bit off-script on this one. I thought there might be moist air blowing on this spot from the evaporator cover vents. I actually think that the vents on the side are probably for intake, but I gave this a shot. I took some plastic, and taped it to the back of the fridge, blocking the spot that ices up from the evaporator cover.

This had no effect.

I then did the same thing and closed off the water filter from the trouble spot.

These are not hermetic seals, but I expected to see some condensation build on the plastic, if this were indeed the source of the problem.

Anyone have any ideas?

Is the spot supposed to be this cold?

What could be the source of the moist air blowing onto it?

The seals look okay. That is, except for the front seal between the doors. There's a torn section I noticed a few days ago, but I've taped it up for now.

I asked this question on another forum, and it was suggested to me that the issue might be compromised insulation. Still trying to learn what that means.