Thanks for responding.
It can write the logs, via the fsGroup settings. It looks like it will fail, but actually it won’t. Even a [ -w <dir> ] test apparently returns true.
Magic, witchcraft, …
Though it’s not explicitly written, the same directory is presumably EFS/NFS “mounted” elsewhere, simultaneously to ES writing logs to it, I guess so some app can read the logs in close to real time, as @buitcj characterized this unusual setup is for “app specific reasons”.