One possibility to look into would be a race such that the stack reports as COMPLETE over RPC but when the parent stack then reads the outputs (which, IIRC, does not go over RPC) they're not yet available.
One possibility to look into would be a race such that the stack reports as COMPLETE over RPC but when the parent stack then reads the outputs (which, IIRC, does not go over RPC) they're not yet available.