I had to replace LCD, Touch, Battery, earlier for a repair, and last the fan (I also had to resold the left speaker wire, first time soldering just in case it helps). When it happens I get sound, can move joycon, and the fan is spinning. After a few seconds I can press power and it goes to sleep mode, then I can press power again and it goes back to normal. A few seconds later it’ll happen again.
Compared to a video from digitalfoundry, the temperatures are okay, fan speed as well (I tried increasing fan speed but it still turned into black screen)
Here’s a couple seconds before the screen goes black: