The only fix i have found to this is to kick the party member from your clan. Then go to the nearest settlement and they will be in the tavern.
Good News:
They will have the latest gear that they were equipped with before you kicked them. (Not 100% confirmed, my companion did)
Their stats will remain the same from before you kicked them. (Once again, not confirmed. Only tested twice)
Bad News:
You will need to pay for them to rejoin your clan. My companion increased from around 800 gold to 1900 gold possibly due to the more expensive equipment he had on now and/or level ups.
The troops that were accompanying them for the quest are lost.
Note: This can happen again to the same companion and other companions. I am not sure which quests in particular are more susceptible to this bug. I only assign my companions to "Land lord needs access to Commons" and "Landlord needs herd delivered to ..." but it will randomly occur between the two for me personally.
From what it sounds like, it may be an issue with the code involving the companions rejoining the party, not the quest itself but its only theories.
Hope this helps. Happy hunting!