Not sure if this is your problem but the quest seems to glitch if you talk to Maven Black-Briar about it before stealing the papers. If you speak to Maven about it the "Steal Frost's Lineage Papers" subpoint is set to done but you do not get the new subpoint "Steal Frost" and therefore you also don't get "Return to Louis Letrush" when you steal the horse (which points to Letrush who is north-west of Black-Briar Lodge in the woods). Not sure if it is possible to fix by stealing the papers from the lodge even if the point is set to done after talking to Maven (tip, use the side door). I just tried to load and not speak to Maven, stole the papers and got the new subpoint to steal the horse, went back and spoke to Maven and then stole the horse and delivered it to Louis.
