We do not have full lerna/monorepo support. We will only cache the node_modules that we install automatically before your build command runs, which would not include any not included in the root node_modules file.
You might want to explore using our monorepo support with the base directory set to packages/modern-agency to see if it works better:
But otherwise, yes, we don’t intend to cache those modules with your current configuration; only the ones we find in the /yarn.lock