I was playing in a world I made in 2D but a bit away from spawn I end up finding the land still goes 3D. At first I thought it was just an issue because I originally started with a bunch of mods, but later on where the only other mods were rendering mods like Sodium and Iris and a mod to remove durability (added after I found this out), it still generated 3D terrain a while from spawn.

i think its likely its the durability no clue why tho,
i had similar issues with other mods
i can confirm it works with these mods on 1,21,1

Oh and if you are adding more mods along the 2dMinecraft mod to you´re world, i would insist you take frequent world backups

Hey! I've just tried to reproduce this bug by going a long distance from spawn, but the mod works completely fine for me (I've got ferritecore and sodium installed).

Can you provide more details, for example your seed, log files and how far you traveled?

Thanks