The normal jar on the maven is deobfuscated, causing all kinds of problems when used in a dev env with a different mapping!

I suggest adding

jar {
    // Do whatever you need here...

    finalizedBy reobfJar
}

to your build.gradle.

Also see MinecraftForge/ForgeGradle#440

I don't know which versions this affects though. Might be a lot though.

@progwml6

Does this affect Tinkers Construct as well, or just Mantle? I noticed a few issues relating to this on the Tinkers Construct issue tracker and I'm wondering if it was just a dependency error

Might be the same issue over on Tinker's Contruct too.

But @KnightMiner this only affects the builds on maven and therefore only affects dev envs with different mappings.

Yes, those are exactly the issues Knightminer is talking about. People using TiC in dev-envs with different mappings (in their own projects).