When you enter the Rivendell Hills, you get the achievement for entering the Lone-lands and you unlock the waypoints for the Lone-lands region. Also, when entering the Rivendell Hills, you do not get the achievement for entering Rivendell, nor do you get the waypoints for Rivendell. It would seem that the Rivendell Hills are assigned to the Lone-lands region instead of Rivendell.

MilkMC ^{Check your render distance, Fandom User!}