Today I added these four mods (in the screenshot, except for Cinematic Music Overhaul). I enabled them and noticed that NO mobs were spawning except worms (for fishing). I checked in different biomes, tried restarting the game, and summoning a blood moon, but no mobs were appearing.
I disabled those four mods, but the problem persisted.
I checked Reddit, and a lot of people were complaining about Calamity Vanities. I deleted that, too, but the problem still persisted. I created a new world, and mobs spawned in it, so the problem is purely with the old world. I switched the mob spawn flag a couple of times with HERO's mod, re-logged, and still nothing.