Hello! I'm here with a very strange issue that I'm not sure how to fix, as I haven't found anyone else running into an issue like this.
Here's a collection of screenshots we took of our world so you can have a better understanding of how the world got corrupted: https://imgur.com/a/4EJGIKm
For clarification, we've tested using these same mods on single-player worlds and nothing corrupts in the same way as when we try to use our server hosted on Aternos. We've also tried regenerating the world and we get the same corruptions. We did also see if removing the certain mods changed anything, in the overworld at least, and when we removed any biome-editing mods that add new blocks (Oh the Biomes You'll Go and Terrestria), but biome-editing mods that don't add new blocks and just use existing blocks in vanilla minecraft to generate new worlds work just fine. So we know there's something wrong with the biome mods that's happening on Aternos's end.
We also know this isn't because we tried updating the world, because we did not, we've been playing with Fabric 1.16.5 and used that same version when loading the world. There's no reason for anything to have updated the way they did as far as we can tell.
When myself and the other person playing on this server logged on for the first time in... about 2 months, give or take, we notice our world had become corrupted in some way as many of the blocks had changed to things like waterlogged stairs and other random blocks. We also cannot collect any of these blocks in survival, though we can use creative selection using middle-mouse to select the item that these blocks had become corrupted into. Chunks that had already been generated had these issues, as well as chunks that had never been generated before. We also found that some chunks we had been to before had strange chunk borders that had never been there before, as the biomes might've been regenerated for some reason.
We aren't sure what to do or who/where to reach out to address these issues, so any and all help would be appreciated!