Description:
You can not delete a non empty folder on a samba share.
This is a known bug and has been fixed in Samba 4.8.5.
My understanding of the issue is, this is due to an error in smbclient. Which explains why I didn't notice the error until I switched my last Windows system to Solus. (Windows can delete a non empty directory from a Solus samba share. But Solus - Solus can not).
Bug 13204 - rmdir on non-empty directory fails silently
https://bugzilla.samba.org/show_bug.cgi?id=13204
Fixed in Samba 4.8.5
BUG 13204: s3/libsmb: Explicitly set delete_on_close token for rmdir.
https://www.samba.org/samba/history/samba-4.8.5.html