Most probably because you do not have "write" permissions on the parent directory of "x".
This happens even for root because the effective security style of the volume would be NTFS at this time.
You can check this with the command " fsecurity show /vol/mixedvol" (substitute the appropriate volume name here)
The solution is to put correct name mapping in place via /etc/usermap.cfg.
Setting the option "cifs.nfs_root_ignore_acl" to "on" may help for root user's access to the files.
You may also review the option "cifs.preserve_unix_security" and see if it applies here.
Check with "man options" for more details.
If this post resolved your issue, help others by selecting ACCEPT AS SOLUTION or adding a KUDO. Cannot find the answer you need? No need to open a support case - just CHAT and we’ll handle it for you.