Frame's contents are not visible on a form at design time when the frame is installed as component

• Lazarus/FPC Version: Lazarus 2.3.0 (rev 60822) FPC 3.2.2 x86_64-linux-qt5
• Operating System: Manjaro Linux + KDE
• CPU / Bitness: x86_64

What happens

A package has a custom frame with 2 buttons. It is installed to the component palette and can be added onto a form. The 2 buttons do not show at design time on the form while the package is installed. Uninstalling the package changes the situation and the buttons show.
At run time the frames's contents show and work correctly. Also when opening the original frame from the package into the designer, the buttons show.

What did you expect

Components of the frame should show on the form always, regardless if the package is installed or not.

Steps to reproduce

Install the attached Package1. It adds a custom frame "MyFrame" on "Sample" palette tab.
Drop a MyFrame onto a form. The 2 buttons embedded in MyFrame are not visible.
Run your project. At run time the buttons are visible and work.
Uninstall Package1. Restart Lazarus and the buttons are visible on the form also at design time.