This occurs only if you encrypt via the button "Sign/Encrypt" in Kleopatra, when you start via the Explorer the selected files and folders are both encrypted.
After clicking "Sign/Encrypt" in Kleo, select some files and folders.
Note that only the names of the files and not the folders are listed in the field "filename" at the bottom.
If you proceed with "Open" only the files are added to the encrypted archive.