It can be a bug, because I've seen a crusade getting stuck in Scotland until like ten turns later when my port was built.

Or possibly that the port became destroyed after he had moved to Scotland but before he could use it.

Or that the comp can see your prodution cue and finds that the shortest route is to wait for the port to be built.

Yes I'm only guessing, but I think I know what's causing the crusades wierd pathfindings.

The crusade can only use the province owners shiplines and that includes you own crusades too.
One of my own crusade had to take an unplanned extra route before I found this out.