1.18 completely changed how chunk iteration works, moving most of the iteration into an off thread that asynchronously updates the chunk graph, and separating out frustum culling into a separate function that can be applied to that chunk graph on the main thread, but Iris for 1.18 appears to have not been changed at all to account for this. This works with Sodium because Sodium overwrites that rendering pathway.

Thanks to @qouteall for pointing this out.