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