I set the end gateway block with exit portal location:
/setblock ~ ~1 ~ minecraft:end_gateway 0 replace {ExitPortal:{X:0,Y:4,Z:0}}
, I teleport to extra post location in X:-5, Y:4, Z: -5 instead of X:0, Y:4, Z: 0. It may mess up your location use "{ExitPortal}".
I think this is intended. See this topic for details on the portal. Also, the tag is called "Exit*Portal*" for a reason.