Put the summary of the bug you're having here

What I expected to happen was...:
When we updated our server to 1.20, we expected to keep all of in-game progress. Outworld and Nether included.

What actually happened was...:
All of our Outworld progress stayed the same, even the things we built hours before updating the server, but all of the building and placed blocks inside the nether - including chests and minecart rails, and blocks we had placed days before the update - have been completely wiped. The Nether is back to the way it was before we ever went in. I double checked the coordinates for our portals in case they were just moved, but they are in the same place. 

Steps to Reproduce:
1. Start a server on 1.19.4 (Or whatever the most recent release before 1.20 was)
2. Go to the nether and build anything. 
3. Update the server to 1.20 and see if the things you placed in the nether are still there.