Turning on oven's fan for preheating - Oven turning off by itself - How to preheat properly
After so much trial and error, I found the solution, and yet, it's so simple:

Leave the oven door open for 2 minutes when preheating, then close.

And I also turn the fan on for preheating, I think it will make it more effective.

It doesn't turn off by itself anymore.