I'm going to test again later from home, but that was not the case. The exact steps were : R, G, B linear images, just after calibration and just after STF to see what I managed so far.
I saved the images on the disk, then I saved the project and then I closed PI.
Next day I reopened PI, loaded the project and the images come up stretched. Really stretched.
I closed them and reloaded them from disk and they were ok.
But as I said I'll test again from home later and report.