Only thing I could imagine would be you told maven about it. This results in the task of taking the paper being checked as done. Normally you still have to get it, though, so I have no idea. "E" tells me you're playing on PC, I'm sure there's a console command to fix the issue.
I came here because I fear the quest didn't work properly for me. I had been telling Maven about the horse and she told me to proceed as planned. So I went and stole the papers, met Louis Letrush and ... nothing happened. ??? Shouldn't Maven interfere or something? It's also a bit weird I ended up with Frost, which went like this: Before Louis got on the horse, he ran off a bit to kill a wolve. He then returned and took the horse, but since I was curious about what to do, I followed him. A while later, we ran into two bandits attacking a traveller (one of those bards you can meet on the roads) and Louis got off the horse, so the two of us battled and defeated the bandits.
Afterwards, he just ran off into the distance, leaving Frost and me behind.
???