Checkpoint prepare will log progress messages every 20 seconds. See here.

A final prepare message is only printed if a previous progress message was logged.

    /* Log the final checkpoint prepare progress message if needed. */
    if (conn->ckpt.progress.msg_count > 0)
        __checkpoint_prepare_progress(session, true);

We should log a final message regardless so that we know the number of dhandles collected/skipped even if checkpoint prepare runs for less than 20 seconds.