When adding a new document to the same instance of a RichTextEditor, the UIProviderRegistry instance of the RichTextBox will keep a dictionary entry for a SectionLayoutBox that contains a reference to a non cleaned up WaterMarkUiProvider.

This SectionLayoutBox in turn has a reference to a DocumentLayoutBox (parent) which has a reference to an old document instance.

A new dictionary entry is added and (retained) for every document loaded.

Disposing the old document makes no difference.