In version 3.18.0 on Linux and Unix systems, when the SFTP server-side creates new files their permissions are filtered against the configured umask.
This was a regression introduced in version 2.8.0, in which the umask value was not used for the newly created files.
An upgrade to 3.18.0 is recommended for any older version running on Linux or Unix.
An upgrade for the Windows version of SFTPPlus is not required.
You can check the full release notes here.