So I loaded up my save from inside the Ratway and after I fast traveled out of Riften, I noticed Durnehviir was on the ground following me. He wouldn't fly away or despawn. He followed me like a regular follower and would still attack enemies. When I went through dungeons he would still be waiting for me when I came back out. I even went to Solstheim and he came with me and was stuck on the boat when I arrived. He ended up teleporting to me when I got too far away. What could have caused this?

How did you do it? Did you just summon him just like anyone else would? Try and use the shout again that should fix it.