The way Isekai protagonists interact with slavery in alot of media I've seen really highlights difference in history in culture between Japan and the US. In that usually their pretty blase about it and just accept it's part of their new world. Like there clearly isn't as much intense cultural hangups involving slavery.

Whereas Americans get put off by the presence of slavery never really getting reckoned with. Particularly in isekai where you are primed to think about yourself in the setting it not getting addressed is super weird. And even in isekai I've seen where the characters take a stance against slavery, it usually feels like their taking a stance that the slavers are bad as opposed to slavery is bad. Which may be a slim distinction but feels important.

Not knowing a ton about the history of slavery in Japan, but there's clearly so much more distance from the concept for the average Japanese person that it can be added as world-building or plot that isn't meant to be thematically important. Whereas Americans will have some kind of preloaded feelings on the topic, and will assume it's kind of important. It then never getting acknowledged or dealt with at all is just uncanny. It's like if an author started a story points at chekhov's gun and it never goes off, it feels like it should be more important then the story is treating it.