LotR -- Book 5 - Chapter 02 - The Passing of the Grey Company

[Kuruharan]

Wild thought: having sworn their oaths, perhaps Eru has stepped in to temporarily rescind the gift until they make good on the oath

I'm not too sure about that. Doesn't Tolkien say that Eru only directly intervened in the breaking of Numenor and the return of Gandalf? Keeping the spirits of the dead in Middle earth was not something the Valar were empowered of themselves to do (although they apparently could.)

As I said, it is a very problematic part of the story.