I think this is a Bug (0.8.5):
When I want to fix the calibration PS has to use the initial parameters. But if there was a previous self calibration (without Fix Calibration -> initial Parameter != refined parameters), in a subsequent optimize process with Fix Calibration PS seems to use (and fix) the refined parameters from the previous self calibration -> the refined parameters after the optimizing WITH Fix Parameters are not equal to the initial parameters!
So I have to re-align the photos if I want to use fixed camera calibration values.