QmlDesigner changing color from solid to gradient and using alpha works always on the second try

Steps to reproduce:

• open/create Qt Quick 2 UI or similar and switch to Designer view
• select a Rectangle item (or any other where color can be used)
• unfold the color properties to see the HSBA values
• by default it should be solid color - if not choose it (alpha should be at 255)
• change the color to use a gradient
• double click on the alpha part (255 should be selected)
• try to type a different value

Current behavior:

• color switches back to solid, value is not entered to the alpha

Expected behavior:

• value for alpha changes

Note: If you now switch back to gradient and try again to set alpha in the above described way it works fine.
But if you switch afterwards back to solid and to gradient again the same behavior as described above comes back - so it seems to work always on the second try.