I haven't updated yet... I tend to wait while things like this are exposed.
I did set up a test however in 12.0v2 and it works fine.
I wonder if it is related to the web viewer or just to that function. I wonder also if you scripted it and applied a script trigger to the field if it would work using variables to capture the data. What you are basically asking the web viewer to do is refresh every time you click in one of the portal fields. The other methodology with scripts would focus on the field then force the web viewer to do a fresh call.