Portal was created in the overworld got to the nether. Upon leaving the nether through the same portal generated on entry a new portal is generated in the overworld.
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This is probably just because portals don’t generate in the nether exactly where they are supposed to. If, for instance, the place where one “should” be is over a lava lake, it will instead generate on a nearby part of the landscape, which can mean that it goes far enough away to generate a new portal.
In short, it isn’t a bug, just a potentially annoying feature.

Its not that. i will make a portal in the overworld, step in and can literally turn around to the portal generated in the nether and end up in a new(not user created) portal in the overworld a chunk or 2 away from the original one I created.
The above comment of mine was in error. I just realized i had to be 1064 blocks away in the overworld for a new nether portal to be generated. not 8 chunks.

Thank you for your report!
We're tracking this issue as MCPE-39609 which has been resolved as "Working as Intended", this ticket is being resolved and linked as a duplicate.
There are some intricacies to portal linking that can sometimes be misinterpreted as a bug. The mod team here as well as Mojang have done significant testing based on the reports that have been made and are unable to reproduce unexpected behaviour.
Please head over to the linked ticket which has a bunch of information in it to help with portal linking issues.
If you believe there to still be a bug, please feel free to head over to the Mojira Discord and discuss it with us in a relevant channel.
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Also portals in the overworld link to the same nether portal even when 10 or more chunks away .