For example, if i did this:

/setblock ~ ~ ~ end_gateway 0 replace {ExitPortal:{X:~,Y:~7,Z:~},ExactTeleport:1}

I would expect to teleport 7 blocks on the Y+ axis (up).
Instead, I teleported to 0,7,0
The ExitPortal tag completely ignores the '~' and interprets the relative coordinates to be static world coordinates