[main/WARN] [mixin/]: Reference map 'undergarden.refmap.json' for undergarden.mixins.json could not be read. If this is a development environment you can ignore this message

Minecraft 1.18.2, Forge 40.1.19, The_Undergarden-1.18.2-0.7.0.jar

this isnt something you really need to worry about seeing, but its been fixed in later versions regardless.