Put the summary of the bug you're having here
What I expected to happen was...:
A new portal would generate at the correct coordinates
What actually happened was...:
No portal generated and I fell through the air into lava before I could do anything
Steps to Reproduce:
1. Enter the Nether through a portal in 1.15.2 and then go back through it
2. Update to 1.16.1
3. Reset the Nether
4. Go back through the portal
Also if a block is placed where the portal should be, the block will be deleted if a player goes through the Overworld portal.
EDIT: I am not able to reproduce the bug explained in this comment.