Designer ContainerControl previews don't show Button symbols

In the desktop designer, a Button inside a ContainerControl loses its Symbol in previews. When a ContainerControl instance is placed on a Window (or nested inside another ContainerControl), the designer preview renders its Button children with their text caption only; any Symbol set on the Button is not drawn. The Symbol displays correctly in the ContainerControl's own editor tab, on Buttons placed directly on a Window, and in compiled apps — only the embedded container preview is wrong.