This is herobrine level glitch that I faced today. I made a portal at -606, 59, 172 coordinate and spawned according in nether but when I tried to return to overworld I was spawned in a literally different coordinate with a new portal which was not built by me at coordinate -470, 70, 139. Seed is attached as image below. Caves and cliffs experimental bedrock edition
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Please see the linked report for a brief explanation of portal linking. If you believe your issue is different, please provide exact coordinates of all of the portals and how they seem to connect.

Thank you for your report!
We're tracking this issue in MCPE-39609, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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The thing you mentioned happens when there is an another nether portal nearby. In my case the second overworld nether portal was never made but it still happened. I still remember correctly the second nether portal in overworld was never made