CPanel FTP users and chown "invalid user"

We're moving from an old server on Plesk 8 to a new server with CPanel (since Plesk 9 and 10 are virtually unusable). Everything seems to be going mostly ok. However when we set up folders in SSH and then configure users for them through CPanel (let's say one of the folders is /home/mainuser/data/collection/ftpuser/), we discover that the folder is still set to root and can't be FTP'd to (though connecting works just fine). And when I try to chown -R ftpuser ftpuser to fix that, it says that ftpuser is "invalid user".

Things I've tried so far:
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Using ftpuser@mydomain.com instead of ftpuser (we have a static IP for this particular domain, but just in case)
Looking in /etc/passwd to see if the user was listed there (it wasn't)
Rebooting the FTP server (Pure-FTPD)
Rebooting the SSH server
Rebooting the entire server

The core problem seems to be that the users configured through CPanel are not showing up in the /etc/passwd list. Why?

 

I forgot that I could just chmod to 777 and have it work regardless of user. I tried doing that, and noticed that when I put in a file, the file got set to the user for that particular domain (mainuser) rather than the FTP user (ftpuser). So I tried creating a new test folder (/home/mainuser/data/collection/ftpuser2/) and setting the owner to the domain user (chown -R mainuser ftpuser2), then setting up an FTP user (ftpuser2) for that folder through CPanel. It worked, so as far as I can tell. I can log in with the FTP user and put in files, and I don't have to chmod to 777. As to why CPanel doesn't create the folder itself, I don't know. Does the folder specified have to be in an area owned by the domain user (mainuser), or is that process run by root and something is going wrong, or what?