The issue as I understand it was that the works were actually in the public domain from the time of death. Copyright as life +75 (or whatever figure) is dependent on copyright assertion in the first place. While we tend to think of this as a foundational truth now, to struggling pulp writers in the 1920s this was often the last thing they thought about. Lovecraft died without heirs; Derleth represented himself as the copyright assignee, without actual legal basis. Nobody cared enough to check for a long, long time, and in truth sorting this out would be a Herculean task. (Can you imagine what the "recordkeeping" of Weird Tales looked like? The stuff that survived? You can get an idea from Syracuse University as part of the Street and Smith collection, https://library.syr.edu/scrc/collections/areas/pulp.php.)
I'm also thinking Conan is in a similar position owing to the circumstances around Howard's career and death.
I'll look tomorrow...
