Hello,
sorry for the late reply. In Edit PDF of PDF24 Toolbox, Opacity always applies to the whole shape, fill and outline together, so the outline fades with it.
A frame with only an outline works online: https://tools.pdf24.org/en/edit-pdf - there, rectangles and circles are drawn as an outline without fill, and the small menu at the selected shape has a No fill button.
Thank you for the suggestion to set the opacity of fill and outline separately in the Toolbox, we will add it to our list.