Hi, this happened to me recently. After going through the nether portal which was located in a Soul Sand valley, I got teleported to the Overworld coordinates of where my portal was. I had to go back to my base which was 1 thousand blocks away. Really annoying. Same thing happened to me with the end portal. I defeated the ender dragon in creative, decide to go threw the portal in survival, and it Teleported me to the bottom of the void. Thank god it wasnt my actual world, that couldve been bad.