Because an AI bot is really stupid. You must explain that in a slug can be a hyphen, and even give an example. If that model is really hard headed you may list every such category slugs.
It is like 3 years old kid. If you explain everything given simple rules to follow, where is no escape routes or doors to start guessing, it works decently good. Most of times. Well unti someone updates that model and it breaks something again.