Steps to reproduce.
Set game difficulty to Hard.
Build two nether portals with the bottom row at y level 0 and the other at y level 1.
Set the gamerule randomtickspeed speed to something higher such as 1000 to cause zombie piglins to attempt to spawn.
Watch as the zombie piglins only spawn from nether portals at y level 1 or greater (going into the positive number range) where as any nether portal at y level 0 or lower (negative number range) doesn't spawn any.
This does not happen on the java edition of the game which I've tested. Zombie piglins spawn from the nether portal regardless of the y level the portal is at.
Link to youtube video showcasing issue: Zombie Piglins Nether Portal Bug
Thank you for your report!
We're tracking this issue in MCPE-149718, so this ticket is being resolved and linked as a duplicate.
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