When copying from the root directory of a drive to a folder (non-root directory on a different drive), this can happen.
RoboCopy may set the new directory to hidden, as it copies the system attribute of the root folder of the drive over to the new folder.
You can prevent the new directory from becoming hidden by adding the /A-:SH
For example:
Robocopy D:\ C:\D_backup /A-:SH
However, if you already copied the directory without the /A-:SH option, running the command below will fix the issue by unhiding the directory. For example:
attrib -h -s C:\D_backup
Ref: How to preserve file attributes when one copies files in Windows? – Super User