When reaching the nether portal another portal links to, continuing to stand inside of that portal causes the screen distortion and nether portal cube overlay to persist, despite the fact that this portal is currently incapable of actually teleporting the player in its current state and the player must first exit it. It seems that this does not always happen upon the portal's first creation, however.

Judging by this video of v0.12.1 alpha, this bug may or may not have always been the case, as the portal animation can be seen stopping as expected: https://youtu.be/5N0x7X-nkCQ?t=4m25s

Steps to Reproduce:

1. Build a nether portal
2. Enter it
3. Stay in the exit portal

Observed Results:
The portal animation still plays.

Expected Results:
The portal animation should not play.