The bug

Non-player entities going through a Nether portal do not generate a new portal in the Nether if none exists. Instead they are just placed at x / 8, y, z / 8, which could be in or above a lava lake as well.

How to reproduce

1. Create a new world, or make sure there are no nearby portals in the Nether
2. Create a Nether portal in the Overworld
3. Throw for example an item through the portal
4. Enter the portal
   → The item is very likely not at the position of the portal

Note: You can also summon an armor stand in the portal to see where it was teleported.

/summon armor_stand ~ ~-3 ~ {Invulnerable:1b,NoGravity:1b,Glowing:1b,Tags:[teleportMarker]}

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

While for players the method net.minecraft.world.Teleporter.placeInPortal(Entity, float) is used which creates a portal if none exists, non-player entities only use net.minecraft.world.Teleporter.placeInExistingPortal(Entity, float) which assumes there is already a portal. It looks like they could use Teleporter.placeInPortal(Entity, float) instead as well without any downsites.