I am guessing that a dead end need to originate in an area. I had one I created that started out of an area, it did contact pathway but it was not where the pathway ended, it was in the middle somewhere. When trying to run a plan I got a general “failed to calculate map” or route, I forget the exact messaging. But there was no detail on why it failed. I guessed that it was the new dead end I tried adding, so I deleted that from the plan and it then would work.
There needs to be more specific messaging, like, “Failed to calculate map, dead end 24 is not reachable” or something like that. I was lucky in that I only made that one change, but if I had made a number of changes or a number of dead ends I might not have know how to fix it. Bonus points if it somehow notes the offending elements on the map, like make the dead end shaded red or note it in the dead end list etc.