This is a bit hard to explain, but here goes.

Back in pre-TU46 on Console Edition, you were able to access the top of the Nether and build up there. Post-TU46, you were not able to move around but could still teleport around with Ender Pearls. After a later update, all ender pearls spawned above y=128 are terminated instantly, and blocks above 128 can no longer be placed or destroyed. Entities cannot interact with blocks up there either, meaning that you cannot destroy or alter anything above the top bedrock on Console Edition.

Pre-TU46, I created a Nether Portal above the top bedrock in the Nether. On Console Edition, no Nether Portals above y=128 could ever be exited, only entered to return to the overworld. I assumed this would be problematic when converting to Bedrock Edition as the Nether's height limit is 128 instead of 256, but it just erased all the blocks up there.

Problem is, any Nether Portal I place in the overworld on Bedrock Edition that's above y=128 now links to the exact spot where the Nether Portal used to be on Console Edition. I can't destroy it on Console Edition because there's no possible way to. Believe me, I tried glitching a ghast into the bedrock to break the portal. It didn't work.

I can't destroy the portal on Bedrock Edition either, since no blocks actually exist up there. The only way to even prove that a portal exists up there is to use one from the overworld that teleports me into an empty field.

To Fix

The easiest fix would be to disallow any portals from teleporting a player into a position above y=128 in the Nether. The more solid solution would be to attempt to find why the game still registers blocks placed above the max height in the Nether after Console Edition conversion.