First of all, I know there have been other reports of this, but none of them seemed to be for the Java Edition, so I wanted to report it anyways.
A chunk in me and my dad's world reset. Summary says "at least one" because there may be more that we haven't found yet. Untouched 1.17.1 terrain right in the spawn chunks, surrounded by chunks originally generated back in 1.2. It appears to have happened a few weeks ago (based on looking at the world backups we have saved), so unfortunately we don't have the server log from the day it happened. The server is unmodded and so is my dad (who runs the server)'s client, although my client has OptiFine, if that matters. Attached is a picture of the chunk, we chopped down some of the trees before I thought to take the screenshot.