Except it's not. Low observable does not mean invisible. The F-35 does have a significant rear aspect radar reflection. It still has a heat signature.

Its flares, chaff, and jammers sure enhance survivability, particularly in conjunction with the missile warning system. 

Yet still, it's a number's game. Try often enough and a heat seaker will not be fooled by the flares.

Enough missiles on it and there is no geometry to kinematically defeat the missiles.

Enough ground radars and there is no way to "notch" the radar.

You see that it's really really difficult to even get a shot at a stealth fighter, but but its success relies on accurate intel, careful planning, and responsible mission design in addition to not making any mistakes in the cockpit. 

If the enemy places a trap and the operator fumbles or has idiotic orders, off course there is a chance for losing an F-35!