Same Issue: Windows 10 Beta - v1.17.30.24
First visit to nether was fine. Second visit replaced all chunks surrounding portal area with flat bedrock at y64, the rest of the nether was fine. After exploring the rest of the nether for a broken portal to return I eventually exited the game while still in the nether. Reloading the world again replaced all nearby chunks with flat bedrock. After dying in the nether, reusing my original portal now places me floating/falling below the flat bedrock into the void.
Is it possable to delete or regenerate the nether, a la deleting dim-1?
Edit: platform/version
Windows 10 Beta - 1.17.30.24
Flythrough video of same issue: Link