The MC-149777 fix appears to cause the error it's supposed to be a solution to.

This only seems to occur when entering the Nether or other dimensions, while the Overworld remains fine. Switching the fix off ironically fixed it.

It's entirely possible this is an incompatibility issue with another mod I have installed, but the only one I could think of that messes directly with world loading is Magnesium. Another possibility is an incompatibility related to Java version: I'm using the Microsoft build of OpenJDK, if that helps, which comes with its own runtime as well.