The bug

If a rideable entity is currently inside of a nether portal and not being teleported, and the player rides it, then they will also not be teleported. However, dismounting this entity and ending up inside those same portal blocks will begin to teleport the player.

How to reproduce

1. Place a boat in a nether portal
2. Go through the nether portal
3. Enter the boat
4. Dismount

Expected results

Either the player would be teleported immediately upon entering the boat, or would not begin teleporting after dismounting it, until they exit and re-enter the portal blocks.

Actual results

Teleportation does not happen when entering the boat, but does upon exit.