Why could this have happened?

This hero is available for hire in the tavern, which triggers the corresponding message when the player visits the dungeon.

and can this be fixed?

Have you tried replaying the campaign scenario?


UPD
I was able to reproduce the problem. This happens if the player hires a hero named Sorcha (also known as Empress). Because of this, she becomes available for hire in the tavern (not available for building) in the 6th mission of the campaign, which causes this message to appear when visiting the dungeon.

Reloading the scenario will not fix this. To complete the map, you need to use cheats or replay the entire campaign without hiring the specified hero.