as I understand it, publication creates a new copyright on that copy.
This is why printed classical music is still under copyright, it's not the copyright of the author, it's copyright of that particular copy by the publisher.
So if Disney were to reprint "Steamboat Willie" and re-release it, they would have a new copyright on the new version
yes, this can get confusing and ugly because the publisher doesn't gain copyright on the older work, it's only their 'version' of it that gets the protection, and the original author could give others permission to publish the work as well, which would get a separate copyright.
But anything that's produced has copyright from the moment it's 'fixed' (written down, recorded, etc)