SFTP disconnects if directory owner is not root

Question

I'm on Debian 11, using openssh-server 8.4, trying to build an sftp server for HTML uploads.

The upload destination directory is "/var/www/html" and the username is "sftp-user".
Also, the server domain is assumed here to be example.com.

I changed the "Subsystem" line in the "/etc/ssh/sshd_config" file to

Subsystem sftp internal-sftp

, and added the following line to change port 22 to sftp only.

Match LocalPort 22
        AllowTCPForwarding no
        X11Forwarding no
        ForceCommand internal-sftp

After that line, I added the following line to specify the upload destination directory:

Match user sftp-user
        ChrootDirectory /var/www/html
        PermitTunnel no
        AllowAgentForwarding no
        ForceCommand internal-sftp

I restarted sshd with systemctl here.

The owner user of the /var/www/html directory should be www-data, so I created a group called "uploders" and specified the group to have ownership of that directory using the following command. And added the user "sftp-user" to the "uploaders" group.

groupadd uploaders
chown -R /var/www/html www-data:uploaders
gpasswd -a sftp-user uploaders

Finally, execute the following command to specify the mode of "/var/www/html" so that the group can also read and write.

chmod -R 0775 /var/www/html

And try to access the server with sftp command:

sftp -P 22 [email protected]

However, the following message is output and the connection is closed.

Connection to example.com closed by remote host.
Connection closed.  
Connection closed

However, if I change the owner of "/var/www/html/" to root, I can access it without any problem.
As mentioned earlier, the directory must be owned by www-data and cannot be changed. Is there a way to enable sftp access even when the directory owner is not root?