Since you know that the camera does some sort of a spot metering on face detect then it makes sense that the pic will have a nicely exposed face but then the nicely exposed face does not mean that the other part of the pics will be nicely exposed as well. In your pic, the face takes up only a small area in the frame. The larger area in the frame is generally brighter than the face. Thus if you pull the face exposure up, it blows out the other parts of the pic. This is how all post editing on photos work. That's just the way it is. It does not mean that the face detect and exposure is not working. It is just not working optimally in your example shot.

Like edpaul has pointed out, face detect on a large sensor has its challenges due to the DOF. Small sensor makes everything in the frame sharp and in focus thus it's easier for the sensor to detect faces. I don't know about you but most people buy a bigger sensor cam so they can get better subject vs background separation, i.e. bokeh.

Why don't you try these next time:

1. When shooting portraits, try to fill up more of the frame with the face.

2. Use fill flash or a reflector to even up the lighting
If you shoot enough you learn how to anticipate shots. I don't use face detect and my A850 is generations behind the latest cam Sony is offering but it doesn't stop me from getting candid shots.





The above are shots of my son and daughter a few years back. You should know children don't wait for you. I can still get shots like the above. Manual AF point selection and no face detect feature.







I shoot weddings too and one cannot expect people to pause a wedding just for your shots, no?