This effectively creates a "soft" copyright. The estate can sue creators for "false association" or trademark infringement if they feel a new adaptation confuses the public or dilutes the brand, even if the source material is technically in the public domain.
, founded by Christie in 1955 and now managed by her great-grandson James Prichard, continues to protect the rights for all works still under copyright globally. Timeline to "Full" Public Domain: agatha christie books public domain