Previously rendered chunks in Nether change to full chunks of Netherack

Fresh world.

World Rendered Nether as normal upon entering.

Explored around for over 400 block radius.

Came back to already rendered chunks - they filled themselves entirely
with Netherack when reapproaching. The entire chunk rendered and when I came back to it, the chunk was entirely filled with Netherack top to bottom.

Left Nether to go to overworld - went back to nether - it was all loading as netherack now and i suffocated.

To reproduce:

Go to Nether.

Run around 500 blocks away from your portal.

Go back to Portal.

All Netherack replaces it.

Overworld Portal at 207, 30, -221

World download: https://drive.google.com/file/d/13tJ5FMYSsqAlxfrxAgo4-DoxJAz6zlAS/view?usp=sharing