Error – the post is displayed in all categories, not only classified

To be clear, you created a custom taxonomy, as an example named “genre”, and you created a term in it, say “comedy”, and created some posts that have this comedy genre term on it. You’re saying the “comedy” term is showing in other taxonomies?

No. I created my own sections – categories (as there are for posts), such as dogs, cats, parrots…. Then I created 3 posts, which I put only in the section (category) dogs. But when I open the category cats and parrots, I see these posts here as well, even though they were not assigned to this category in the administration.

So this is for a custom taxonomy? Trying to make sure I’m fully understanding because “category” is a very specific taxonomy that comes with WordPress out of the box, and not a generic phrase.

Good day,
yes, custom taxonomies, sorry. Here’s a video that hopefully clarifies it: https://www.youtube.com/watch?v=UBy1cd-N1q0 – at minute 19:47 you can see the post being added to “Taxonomies” – audi, BMW, Ford. According to this setting, I assume that it should then only appear in the Audi section. But if I open, for example, the Ford or BMW section on my website, I see this post there too. I probably won’t explain it any further, at most I can send a private message somewhere.

Sounds like potentially some customized querying going on if your posts are showing up in term archives that it shouldn’t be in, and that’s not something we have any control over.