I kind of assumed one astral entity could follow another astral entity on the astral even in fast mode. After thinking about it, I now believe if LOS is lost at any time, it would be virtually impossible for the pursuer to relocate the leader if the leader alters their route.

Any thoughts?

Well yes.
Unless the other entity knows where the leader is going or has a linked astral sig to follow. There is a contested magic test, if memory serves, for an astral being to outrun/lose an astral attacker/tracker.

Edit: of course if the leader is friendly to the other being then would wait for and actually 'lead' said being, even in fast mode. As the astral is so subjective in terms of speed as long as the two being choose to stay/wait for one another, no problems. Bar unusal circumstances.