The Nether of one seed is the same as another, and this bug is annoying since it has happened to me in several worlds. Here I tested with 2 worlds: the first (8171674506331821349) at these coordinates 180, ~, 800, when opening the portal it appears in that Nether, and in the second world (1222090162574629), when opening that portal it appears in the same Nether, with the difference that the portal is below instead of above.
We're tracking this issue at MCPE-154947, so I will resolve and link this report as a duplicate.
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