So I came to close to finish Lord of Barren Lands quest line (after talking with the Lord and decided to go after Koslc instead), but need to come back to Stronghold to defend it from a mob of skeletons (since it was very close to the deadline and I need to go back and forth between Kolsc's place and the castle as well). Dealt with the skeletons mob, but there is no cutscene indicated that I successfully defended the stronghold. I just disregarded it and continued to play. But the mob respawned everytime I came back to Stronghold  and/or I rest at Blighthollow . Same thing happened for another bandits mob that I need to kill to defend the Stronghold.

 

Bellow are the save files

 

https://www.dropbox.com/s/dtgfhfhd8ryndqp/123c34d8-c69b-4fa6-a861-209fada976b2%20quicksave.savegame?dl=0

 

https://www.dropbox.com/s/nglherb06vah9qg/123c34d8c69b4fa6a861209fada976b2%2010406586%20CaedNua.savegame?dl=0

I got the same problem, but it somehow got fixed :D Just try to battle them from time to time and continue with the story, it eventually worked for me, now all these 'attacks' work for me