Weird Floating Textures in the Void
Can reproduce with Minecraft 1.21 and following mod list:
- fabricloader 0.15.11
- fabric-api-0.100.3+1.21
- distanthorizons 2.1.2-a
- Create a superflat world with "the void" present
- move to any of the corners of the stone platform
- fly up to around y=120 and see this floating texture in the void

Solution
I think that’s just cave culling acting strangely.
Try the latest nightly build or disable cave culling.
Try the latest nightly build or disable cave culling.