This occurs when you have placed the portals too far apart. They do have a max range, although it is not indicated in any way whatsoever.
Do not wish for the blink update. 99% chance portal will get los nerfed, range nerf, player only Nerf, ooc ends it Nerf, destructible Nerf, enemies can use it too bug, enemy tower guards can use it bug, you come out of the portal upside down stuck in terrain bug and pretty much anyway inconceivable way to make skill bad.
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