Look carefully at the file ownership on the Linux system after the file has been created, and look carefully at the directory permissions and any ACLs of the directory being written in to.
If the directory being written in to is suid or sguid, then the owner or group of the file written would be changed automatically to match the user or group of the directory (respectively). That can leave the file unmodifiable by the original user.
Or the cifs daemon might be writing the files as owned by the cifs daemon instead of the user.
If the user does not have an account on the linux system then the credentials accessed with would be the anonymous user, which would normally fall in to the "other" category. The directory might have "wx" or "rwx" permissions sufficient for the anonymous user to deposit the file, but especially with the before-mentioned suid / sguid possibilities, the anonymous user is not certain to have write access to the resulting file.
If the CIFS daemon is not running suid root, then it might write the files under its own uid, but then be unable to chown() the file to the account of the user, leaving the file unmodifiable by the user.
... and there are other possibilities. Most network file systems are security hacks, trying to balance between competing requirements. You need strong trust mechanisms to do network file systems properly.
