When there are unread feeds, the akregator icon is "empty": it is still there, takes space, and can be clicked, but does not show any icon, just empty space.
The icon appears again when all feeds are read. This used to work in previous versions of akregator.
The icon is displayed for me in 5.6.1, but now it doesn't seem to change at all when there are unread feeds. The window icon is now missing.
it seems fixed here.