How to restore less recent Whatsapp backup from iCloud drive?
Do you backup WhatsApp messages in WhatsApp settings or iPhone Settings? If so, you cannot access or view the backup files in iCloud Drive because it is not saved in iCloud Drive. It is unreachable like your iPhone backup.
As for the situation that you just got one chat history back, I think it is a bug. You can reinstall WhatsApp and try again.