The quest 'Diplomat of the Deeps' asks you to speak to 5 other fortresses. However, parleying 5 times to one other fortress can also complete this quest. (For example, the other fortress which gives quest 'Loot and Lucre!' and you don't have required luxury on hand.) So it's very likely a bug.

Might have to use this to pacify the talk to 10 other fortresses quest... 10 is waaaay too many when you can start getting instant results (resources) just by killing them and capturing the fort.