MMH5.5 - Heroes of Might and Magic 5.5

Hello, has anyone encountered a problem where only a few heroes are available per faction on a map? The map was downloaded from Discord and is specifically designed for version 5.5 (the specific name is H55-8P-World-at-War-1.9). I read the manuals and other resources, and followed all the instructions using MapMixer. All map settings (e.g., dividing players into teams) can be changed easily, but the available heroes remain the same limited set. I've looked through many pages on this topic, but I haven't found a similar problem. Can anyone suggest how to solve this issue?