fg.deobf failing for sub-projects

I've got my workspace laid out as such:

[workspace root]
         |
         |-- LabKore
         |      |
         |      |-- build.gradle
         |      |-- src, etc..
         |
         |-- PowerKit
         |      |
         |      |-- build.gradle
         |      |-- src, etc..
         |
         |-- build.gradle
         |-- settings.gradle
         |-- etc..

PowerKit and LabKore are two individual mods.

I wish for PowerKit to have a dependency on LabKore, and although everything looks fine in Eclipse, when I run runClient, Minecraft crashes with a InvocationTargetException.

Looking in the logs, I see the following lines of interest:

Exception message: java.lang.NoSuchMethodError: 'net.minecraft.world.item.Item$Properties net.minecraft.world.item.Item$Properties.m_41491_(net.minecraft.world.item.CreativeModeTab)'

Cannot deobfuscate dependency of type DefaultProjectDependency_Decorated, using obfuscated version!

In PowerKit's build.gradle, I have added LabKore as a dependency like so:

dependencies {
	implementation fg.deobf(project(':labkore'))
}

Why can ForgeGradle not deobfuscate LabKore? How can I fix it?

Thanks