Not all heroes from the game can be selected at the beginning. How can this be fixed?

I created a map in the editor. I decided to expand the list of standard starting heroes by adding heroes from the campaigns and scenarios. As a result, I encountered a problem because not all heroes are displayed in the list of starting heroes. For example, when creating a game, I choose the Necropolis castle, and in the list of heroes, there is Markl, but Arantir is not there, even though I also added him. How can I fix Arantir and other heroes with the same problem (there are several of them) and be able to choose them as starting heroes?