It was probably a mistake on my part during installation, but for some reason, IFMLLoadingPlugin is loaded by the sun.misc.Launcher$AppClassLoader, while MixinBootPlugin is loaded by the net.minecraft.launchwrapper.LaunchClassLoader. Due to this difference in class loaders, from the JVM's perspective, MixinBooterPlugin does not implement IFMLLoadingPlugin, which causes a ClassCastException in the loadCoreMod method of the CoreModManager class. As a result, Forge interprets the mod as not being a core mod, and therefore it doesn't load in the development environment. I managed to fix this by modifying how this class handles the casting, but I still want to understand why this error occurred, if anyone else has faced it, and what I likely did wrong.

Would like to see the log, are you making a mod with MixinBooter as dependency or are you purely trying to run the mod?

Would like to see the log, are you making a mod with MixinBooter as dependency or are you purely trying to run the mod?

Well, as I was looking through the repositories from this group, I ended up finding the build template, and with that, I was able to fix this problem and a few bugs. I’m not exactly sure why that was happening, but after seeing this build template, I have some ideas. It's a bit embarrassing, but my build.gradle was terrible—lots of useless stuff, issues, and workarounds. Anyway, thank you very much for your attention; I didn't expect anyone to actually answer both questions. Hugs, and thanks again.

Never is embarrassing, I've seen many buildscripts and the nature of gradle makes it unnecessarily complicated. If you have any questions just pop on discord or reply here.