Discovered when testing T165579.
Attempting to add a statement with data type "form" on a form/item/property fails with "Could not save due to an error. [3f83348963955a7463632683] Exception caught: The given ID does not refer to an entity of type lexeme"
Clues:
The SetClaim request calls ReferencedEntitiesDataUpdater::addLinksToParserOutput when saving the property which in turn attempts to do an EntityTitleLookup and calls EntityHandler::getTitleForId on the LexemeHandler with a FormId at which point it breaks because it expected a LexemeId.