A/T temp light came on during long climb - normal?

I was driving up a sustained uphill, maybe 20min drive, gaining about 500m. The road was rough'ish, but nothing hard. Was shifted into 4Low. The 'A/T temp' light came on. I stopped let the engine run for 5min, then turned off the car and turned it on again - light was gone. Is this normal? Should my transmission be overheating? I wasn't hammering the gas or anything, just steady climb up.

This happened earlier this summer when it was 30c degrees. But now it's below freezing and still happened again! The actual engine temp (coolant temp) was normal the whole time, just above the mid-point.

The truck is a 04 Xterra Supercharged. Running a lift with 32s (BFG KO2s), aftermarket bumpers and winch, but the truck itself was empty, so not very loaded down or anything! I had a shop replace the transmission oil about 6 months ago as well, when I got the truck.

I know that on longer climbs shifting down into 4Low is recommended to ensure the transmission gearing stays high so the oil circulates. I did that, maybe I should have shifted down into 1st or 2nd gear as well??
Have you changed the transfer case oil lately? Could be low or old. Have you checked the AT fluid level? Did they put in the correct stuff? Nissan ATF is a specific thing. Also have you change the fluid in both front and rear differentials?
Are any of your brakes sticking ? This could lead to an overheating transmission also.
Any time I’m in 4Lo I am usually paired with 1st for down hill/ rocks or 2nd for up hill and or mud/ snow. I usually use 4lo+drive when I’m just putting along down a tight twisty bumpy trail that I’m in no hurry to find the end of. Keeps me from having to modulate the skinny pedal all the time.
Transfer case and both diffs had the oil replaced at the same time. I really hope the shop knows what they're doing and put specific Nissan ATF oil in when they changed the A/T oil...

Brakes are fine.
That is what the warning light is for. You were in the heat generating, convertor slipping with a bit of load zone. And running for a long time. You were building heat faster then it could be rejected and kept at it. Yes, automatic transmission will build heat doing that. If you were hammering on it you would be more out of the slip phase of the convertor and more into the coupling phase, it would probably be running cooler.

The good part is you headed the warning light and didn't ignore it. In theold days there were no warning lights, you just cooked the trans