In this case the playbook basedir is /group/playbook.yml so I would have expected the other to also be ok.
If you find this is not actually the case, a patch that allows both to work would be accepted after we test it a bit.
Most of our examples are quite simpler though, and just leave the playbooks at top level and say:
roles:
- web
Which I find easier than having to dive into the directory, as the playbooks to me are the interesting thing, and roles are the things that you are likely to have more of.
Up to you though.
–Michael