I found this on Mojang bug tracker that's still unresolved, and reported also for versions 1.17.0, 1.17.2 Hotfix:
This report covers any of the following occuring without a game crash:
- Chunks (portions of the world) being wholly or partially (i.e. below a certain Y-level) deleted and turned to void/air
- Chunks being reset to a state of initial generation (This may result in the chunk not matching surrounding chunks that were generated in an earlier version.)
- Chunks reverting to an earlier saved state (usually the state the chunk was in before the last time that area of the world was visited)
Which seems your same problem.
And a mod's comment states:
unfortunately, there is no way to make the chunks that have been voided come back. The data for those chunks is simply gone. The only thing you can do for future protection is make manual backups of your worlds regularly and restore one if new chunks get voided.