Using a large nether portal can put you on nether roof..
to recreate.
Create a nether portal in the over world larger than 6 blocks wide.
enter portal in the corner. Not the middle.
you should end up on nether roof
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Can you attach the world where you can recreate this? Seed + coordinates of where the portal should be placed would be better

Thank you for your report!
However, this issue is Invalid.
Your game, launcher or server is modified.
If you can reproduce the issue in a vanilla environment, please recreate the issue.
Any non-standard client/server/launcher build needs to be taken up with the appropriate team, not Mojang.
Any plugin issues need to be addressed to the creator of the plugin or resource pack.
If you have problems on large servers, such as The Hive and Hypixel, please contact them first as they run modified server software.
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Given MC-121098 causing suffocation in this case (which, granted, I haven't tested in a while), my guess is that this is specifically a mod-related issue. If you can reproduce it on a vanilla setup, please comment.
Cannot reproduce. Can you please show us a picture of the setup/portal you are using?