Upgraded to 1.17.3 successfully on BDS. However, below y40 all blocks have been deleted and turned into void. I attempted to place a world backup into the worlds folder and had the same result. This has completely ruined our server's map and we are unable to downgrade back to a previous version as most of our members are on console. This map is only 2gb in size and has been running for a few months. Everything above this y level is fine. 

Edit: Further clarification upon examination. Added a second photo showing an area of the world that used to be at around y40 is now y0. Somehow the world has shifted and in a way its deleted the entire lower portion of the map. 

Something in this most recent release has caused the entire map to shift and become unusable.