Setup: main config file with "Include /etc/ssh/sshd_config.d/*.conf" line as the first active directive.  Create /etc/ssh/sshd_config.d/test.conf with:

Match Group sftponly
ForceCommand internal-sftp
ChrootDirectory /sftp

Now, assuming a working chroot layout (/sftp owned root:root, /sftp/home/testuser exists, testuser is in group sftponly and their home dir is /home/testuser), run:

sshd -C 'user=testuser' -T

The ForceCommand and ChrootDirectory are not applied, both according to the test output, and in practice.  Note that no error is generated.

An inverted approach will chroot all users, thus proving that the config itself is successfully being loaded:

ChrootDirectory /sftp
Match Group ssh-users
ChrootDirectory none

Observed in Ubuntu 20.04, and unmodified builds of the 8.2p1 and 8.3p1 releases.  The man page does not indicate this limitation.