My guess is that the jar file didn't obfuscate itself to match what the actual method is (e.g. func_numbers_letter). The command I use is 'gradlew build' which is what I assume is what you are doing. You might need to either try a different version mapping or source version.
Remember you can deprecate methods in the core to be removed later to allow backwards compatibility with older versions if the changes affect a wide range of mods you are creating. If you are including the libraries as a standard utility, you are most likely not going to reuse the code in other projects.
?? that is all false
if you dont know if it has a cap use for interfacing w/ other tileEntitys
energyCapabilitiesExternal.ifPesent(lambda goes here)
if you know if you have a cap there use
energyCapabilitiesExternal.orElsethrow(throw a runtimeexception there)