Steps to reproduce

1. Place armor stands
2. Fly away so that the armor stands are outside of simulation distance.
3. Save & quit.
4. Reload the world.
5. Fly back to the armor stands.
6. Make a nether portal and go into it.
7. Once in the nether, return to the overworld.

Expected result

The armor stands stay on the blocks where you placed them.

Observed result

The armor stands drop down inside the block where you placed them.

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armor stands drop down one block whenever i use a nether portal

So i found out that whenever I'm going back of the nether my armor stands are one block lower every time, this happened in my minecraft single player world, vanilla without any mod or texture pack.