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Selecting in Area Fill Gradient Gradient 6 and then go to Transparency shows Gradient 6. See below
Conclusion is: Whenever a switch was made from Color to Gradient, the Color does not show up in Transparency. The same is valid for Hatching, Bitmap and None.
However, changing to all kind of Gradients, Hatching, and Bitmap, and None works correct when going to Transparency. Only Color does not show up in Transparency when there was once one of the other Fill were selected
I did this test several times and got always the same result.
The only way to get back to Fill Color is to close and reopen the Data Series window.