Did a quick dive into this, as Popeye’s copyrights are public domain in Europe: the Popeye “universe”, as it were, is considered a work for hire, and thus, Hearst is able to somewhat control how exactly Popeye exists in the public domain worldwide.
Modern problems call for modern solutions, which is where trademarks come in. In this way, Hearst is able to trademark the character designs and ensure that they’re the only ones who can still make the “real” Popeye stuff - forever and ever, Amen.
Not to mention, there’s other stuff in the land that belongs to Hearst - Hagar, Phantom, Flash Gordon - and for whatever it’s worth, remember our homie Little Nemo? “Oh, I’m falling out of bed”? He’s public domain, too.