Since v2.20, it appears that CalculatePath no longer works (PathCalculated event handler never fires) if used on a unit that is outside of the map borders. It seems there is a hard cutoff at about 275 meters out of the border where it suddenly stops working.
I don't know if this was part of the optimisation, but it is a regression of an important feature, since all things considered, it is perfectly valid for a unit to be placed outside of the borders. From both a mission design and AI programming perspectives, it is very useful to be able to spawn units outside of the map borders and have them move into the map, as a means to disguise spawning from players, give a bigger sense of logistics to the game and widen the useful area of the map without compromising active areas.
The combination of CalculatePath + setDriveOnPath is an especially useful tool in helping AI navigate around any challenges that may be imposed upon them when they spawn outside of the map, and it has proven essential in my AI programming. The situation I described used to work perfectly until this update.