I get output that looks like this:

substitution of /gnu/store/g26z8hhqagcsiwz611jwhyj7rk6zywqg-python-pybigwig-0.3.17 completesubstituting /gnu/store/slxjk0sg8q85qk0l0dv5kv71z2l4pgv2-python-pybrowserid-0.14.0...
downloading from https://ci.guix.gnu.org/nar/lzip/slxjk0sg8q85qk0l0dv5kv71z2l4pgv2-python-pybrowserid-0.14.0 ...

Also, could the value of (simultaneous-jobs status) change between invocations in the code? It's ok not to use let?
Adding a newline seems to fix it.