This happened on my own OFFICIAL Mojang bedrock server, fully stock, no plugins, no resource packs, no nothing, version is 1.16.210.06, when a video was recorded no one else was on a server, also, not a single one porlal is nearer than 3000 blocks(in overworld) and nearest portal to my portal in the nether is 500+ blocks far away from it.
All started since 1.16.210. As you can see on the video I teleported to my overworld portal, then you can see a lighting bug that exists for 6 month or so, but it's nowhere near as annoying as a teleportation bug. And then i entered the same portal and it teleports me to the same coordinates as in overworld but in the nether!!! And it 3000 blocks away from my nether base, this is SO MUCH in this dimension, especially when you don't have enough food and at least a pickaxe. Also when it teleports you to the wrong coordinates, portal in the nether does not generate, it only appears after you reenter the server. In the end of the video there is a portal generated the previous time bug happened, I always brake it to not have possible problems with it in future

EDIT: I tried to do research and created a BRAND NEW server, started there an absolute new world and this bug also happened there!! It took me some time(I tried about 20-30 times) , idk, maybe it happens more often in bigger worlds such as world that in the video(146 mb) but it happened, please fix it, it’s so bad!!!