In short, if a chunk is not loaded and you will run into it your Y position is decreasing. That means you can be on brige in Nether and when u spet into unloaded chunk, game will teleport you into lava ocean or blocks.
I hope it's understandable and helpful.
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This isn't a bug, but more related to MC-149178 This probably is happening because chunk loading in 1.14 is quite broken. Either that or your PC can't load chunks fast enough, causing you to walk into un-rendered chunks. This happens with every version of Minecraft, you're probably just noticing it now because of how slow 1.14 chunks load.