It's caused by the instancing of the new zones SE has temporarily implemented to ensure server stability (which essentially copies that zone several times), they did it with Heavensward and Stormblood too. Sadly there is no way around it other than asking your partner to move to a non instanced area and then use the ring then (another place that the Eternity Ring will not work is if your partner is inside a FC House, Personal House, Personal Chambers or Apartment, as they are also instanced and thus the ring's teleport function does not work there either).