This sounded far too intriguing to not google a bit. I'm guessing you're talking about this?

> J.R.R. Tolkien first tangled with copyright law in the mid-1960s when ACE Books discovered that the copyrights for The Lord of the Rings and The Hobbit had not been properly secured in the United States by his American publisher. ACE Books opportunistically published unauthorized paperback editions of those books in the United States. Until that time Tolkien’s books had only been available in hardback. Tolkien and his publishers reacted quickly by publishing their own authorized paperback editions, but in order to secure copyright in the United States again Tolkien was forced to revise both stories (thus confusing the issue of what is canonical in Middle-earth for his readers).

> There remained a question over the status of the ACE Books edition until several years ago, when a long-running complaint by the Tolkien Estate was finally settled with payment to the author’s heirs. Until that time some people in the entertainment and publishing industry argued that there was still a copyright “hole” pertaining to Tolkien’s works in the United States.

Source: https://middle-earth.xenite.org/why-are-some-people-sued-ove...

They're referring to Golan v Holder, which affirmed that the US could retroactively restore copyright to works that had fallen into the public domain in the US under the US's old copyright laws, but which remained protected by foreign copyright laws which the US had adjusted to be in compliance with. So basically The Hobbit, like many other works, was copyright protected in other countries, but had fallen into the public domain in the US; when the US agreed to honor the foreign copyrights, it became copyrighted in the US as well.

https://en.wikipedia.org/wiki/Golan_v._Holder

Seems like they are related, from reading https://law.marquette.edu/facultyblog/2011/10/one-public-dom...

Please correct me if I'm wrong, but it sounds to me like in the US The Hobbit never formally was under copyright due to not fulfilling the requirements when published. It remained that way until 1996 when URAA made the UK copyright valid in the US. Golan v Holder then affirmed that public domain works could be copyrighted under those special circumstances.