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.
- Create a new world, or make sure there are no nearby portals in the Nether
- Create a Nether portal in the Overworld
- Throw for example an item through the portal
- 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.
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.