This was a consequence of the work done in #106321, where this specific 'active in UI' case was not identified and properly handled. Now, consider most ID usages from UI (editors) as 'weak links', i.e. keep a reference to these IDs even if they are only indirectly used. Note that missing weak links will not create placeholders if the source data is not found in the library anymore on load. they are just silently dropped. Pull Request: https://projects.blender.org/blender/blender/pulls/122207