On the java nether update the ability to place portals above bedrock was added and I believe it is time to bring this feature to Bedrock edition. The height limit is still 178 on the latest beta build.
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Thank you for your report!
We're actually already tracking this issue at MCPE-18263, so I will resolve and link this ticket 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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This is a parity issue. On bedrock the nether world height is 128 (which hasn't been changed since the overworld height limit increased) while on java it is 256.