Originally reported by violencia

ALL MODULES DISABLED? true
OS, Hosting, Browser (if applicable): None
Short Description of bug: Foundry core seems to ignore a system's registered Folder Config custom sheet
Simple steps to reproduce the bug:

1. Add DocumentSheetConfig.registerSheet(Folder, 'namespace', MyFolderConfig, {makeDefault: true}) to a system's initialization hook, such as init.

• For comparison, DocumentSheetConfig.registerSheet(ActiveEffect, 'namespace', MyActiveEffectConfig, {makeDefault: true}) works as expected.

2. Refresh Foundry to load the changes.
3. Try to either create or edit a Folder through the sidebar.
   Screenshots and/or console errors:
   MyFolderConfig vs MyActiveEffectConfig