How to get the parent of a non-hierarchical custom post type being edited in Gutenberg

It’s a common practice to use the post_parent property in the WP_Post class to create 1-to-n relationships between different custom post types.

In an plugin I’m developing, I added a SelectControl to the PluginDocumentSettingPanel to be able to set the id of a related custom post type in another custom post type. According to the docs, I should use getEditedPostAttribute to retrieve the current “parent” (the id of the related custom post type) of the custom post type being edited:

But, for some reason, I’m getting an undefined value even though the post_parent property is set. A custom meta box in the classic editor shows it correctly.

If I set the hierarchical property in the custom post type to true, the selector works and I get the correct value. Is this by design? Or are we forced to use a custom meta field to store this data, now?