I am running a few servers and tried to update from 1.16.5 to 1.17pre1 with one of my worlds and that went flawlessly. Then up to pre2. Also no problem.
Then yesterday evening i updated my 1.16.5 to 1.17 and then my whole world was reset. All buildings were gone. I used the same java on both debian servers, zulu16.30.15-ca-jdk16.0.1-linux_amd64. I have to admit that while the server starts it looks as if java displays problems but it scrolls so fast that I cannot see anything. I think the need to run java16 is quite annoying, as it is very new and not implemented in many disto's.