When i Alt+tab on full screen mode and get back to the game whit alt+tab put me back at windowed mode 

This is like this since years, but they still not managed to fix it, probably they don't know how or jsut don't care.

I dont have this problem since i started to play ,it come out after the big server crash

Try ALT ENTER, when you ALT TAB out off game and come back to game.

I just read that it will turn back to full screen, try it.

If it works, pls reply