The bug

If the player teleports to the End directly from the Nether either via an execute command or by an existing end portal block, the nether portal animation will appear on-screen instead of the end portal.

How to fix

Display the end portal animation on-screen instead.

Further notes

Alternatively, it might be a good idea to give Overworld->Nether, Overworld->End, Nether->Overworld, Nether->End, End->Overworld and End->Nether unique background textures somehow.