Seed 1537846859 - In Realms from a Microsoft Windows Bedrock client. When travelling through a nether portal from the nether to the overworld for a few seconds different terrain than expected appears, in this case, I appear underwater in an ice biome, this world was generated in 1.17, and checking the same seed for the current version, where the portal is, is an ice biome now, though it appears to be an ice ocean so it could be a corrupted merge between the versions. I suspect that when passing through the portal it attempts to generate the overworld as it would appear in 1.20, not the underground base I built in a plains biome. This results in me underwater with no portal, where I start drowning and if I move too much the client realises it is incorrect and then correctly generates the terrain and I appear in my underground base, but if I have moved I could take suffocation damage from appearing within blocks around the portal even within the portal obsidian itself requiring me to break blocks to escape.  

This also applies in the opposite direction too. I have a portal in a building over a lava lake, occasionally when travelling through the portal, the poral and all structures that I a have built in the nether are gone and I load into the nether falling into the lava, a few seconds later the structure appears above me, but I'm already burning in lava. I have taken to not trusting nether portals and always carrying a fire resistance potion so I don't die.

This happens on my realm from Microsoft Windows client, interestingly, the same portals, on the same realm do not have the issue on Minecraft for Switch or mobile.