Whenever I go into freecam mode, the world stops rendering altogether.
*No, I don't have the two keybinds on the same key
*This just started happening abruptly for no reason
*Using Optifine, no shaders, no Fast Render
So, what's wrong?

(EDIT) It went away after relogging and closing the launcher (using vanilla launcher)

No idea what would have caused it, but it might be a conflict with Optifine, and it might be fixed by the changes/improvements that were made in the 1.12 version, which will come to the other versions at some point.

Same issue in Fabric 1.15.2 with Optifine. Fixed it myself by deleting optionsshaders.txt in my Minecraft root, which is strange, because I wasn't using shaders.