The Bug

When travelling from the Nether to the overworld, it is usually the case that portals outside of the world border are considered invalid, and therefore a new portal inside the world border is created so players are safe.

But this is not the case for outside portals that are outside of the world border by ~128m or less. These portals will let the player use them, and then the player will instantly die.

How to Reproduce:

1. Create a world and input these 3 commands:

   /worldborder center 0 0
   /worldborder set 2000
   /tp @s 1000 70 0
   

2. Build a portal and go to the nether.
3. Input:

   /execute in overworld run worldborder set 1780 

   Your overworld portal is now ~110m from the border, on the wrong side. We had to use an execute command because without it we'd only be affecting the nether's world border and not the overworld's. See MC-121997.

4. /gamemode survival and return to the overworld.
   -> "Player suffocated in a wall."

If you repeat these steps and use 1750 instead of 1780, you'll find that a new portal is created, as intended.