@STerliakov gets the credit for the answer:

Apparently, this section of the code in its entirety was added for the purpose so the user would see the data for troubleshooting purpose.

I deleted it entirely and that fixed the issue

        requirePOST();
        [$uploaded_files, $file_messages] = $this->saveFiles();
        Flash::set('debug','Application form data '
            . json_encode($_POST, JSON_PRETTY_PRINT)
            . "\nUploaded files:"
            . json_encode($uploaded_files,JSON_PRETTY_PRINT)
            . "\nFile messages:"
            . json_encode($file_messages,JSON_PRETTY_PRINT)
        );

For anyone else experiencing the issue, here is the entire working function

    public function submit() {
    
    include([DB ACCESS FILE]);
    
    $fp1 = json_encode($_POST, JSON_PRETTY_PRINT);
    $fp2 = json_decode($fp1);
    $owner_first = $fp2->owner_first;
    $owner_last = $fp2->owner_last;
    
    $query_insert_cl = "INSERT INTO cnvloans (owner_first,owner_last) VALUES ('$owner_first','$owner_last')";
    mysqli_query($dbc, $query_insert_cl);   
    
    
    redirect('/page/confirm');
        
    }