Portals generate far-away chunks & set player on fire

The bug

The anti-cheat engine is being over-zealous causing the following issues with nether portals, caused by the game teleporting a player back:

• Portals setting the player on fire if there is lava at the exact location of the entry portal in the other dimension.
  • MC-97523
  • Can kill player if they are low on food or health - very serious.
  • Deals 11 damage (5 1/2 hearts) over time.
  • Usually survivable in survival mode, especially if you are full on food or have good armour - you will only take 1 or 2 hearts' damage.

• Portals loading chunks at the exact location of the entry portal, but in the other dimension.
  • MC-97523
  • Only the chunk at the exact location of the other dimension's portal loads.
  • Only the terrain is generated: entities do not spawn.
  • InhabitedTime of that chunk is 0.

These issues are usually characterised by a debug message in the chat claiming that the player moved too quickly:

(this was going through a nether portal in the overworld at 10 000, 10 000).

Workaround

Remove all fire/lava at the exact location of the entry portal in the other dimension. E.g., if you have a portal at (100, 64, 100) in the overworld and you are set on fire when going to the nether, go to (100, 64, 100) in the nether (not to the location of the exit portal) and remove fire and lava around that exact location.

Related issues

MC-90062 - which covers the other effects of the cheat engine changes
MC-86850 - which covers how taking certain actions may cause you to remain at the erroneous location.

The fix

A suggested fix by Xcom6000, Timothy Miller, and [Mod] Pokechu22 can be found in this comment.