Deleting the only object under a prefix in a versioned bucket writes a
delete marker and keeps the version history, so the filer directory
survives with nothing a current-version listing would return. A delimited
ListObjects kept reporting that path in CommonPrefixes, because the
prefixes come from the directory tree rather than from the keys, while a
listing scoped inside the prefix correctly came back empty.
Probe a directory before reporting it: one that holds entries but no key
the listing returns is neither a CommonPrefix nor a path the
trailing-slash probe answers for. Empty directories keep the meaning they
have today, and the probe only runs for buckets with versioning
configured, the only ones that can reach this state.