Invalid stat on . was preventing write access to root of share. Add experimental non-default option to chroot the SMB session to the share's connectpath. QE test is create "generic" share type dataset with no ACL on it. Change dataset so that it's owned by the smb user account. Connect to share, and copy a file to it.
Regression was caused by performance optimization to avoid all calls to acl_get_file() and rely on POSIX mode exclusively to create a Windows SD.