Was playing a lan game with my daughter (she was hosting) and after I visited the nether and returned a significant number of the chunks in the overworld were replaced with chunks from the nether but only on my computer. For my daughter everything was correct. This created strange situations where the nether chunks that I could see were solid, but had no effect for her and in my perspective she would fall straight through them. If I fell into a deep section of nether I end up suffocating because I ended up underground on the host machine.

YouTube video of things https://youtu.be/ubdGJr-B_68