D8 | submitForm | Get the $_POST - vars via $form_state or via \Drupal::request()?
Not entirely sure what you're doing, but the answer is probably $form_state->getValues() or $form_state->getValue(). There is also $form_state->getUserInput(), but as you'll see on that page, it recommends you use one of the first 2 options unless you have a specific reason and know what you're doing.
$request->request->all() != $form_state->getValues() $request->request->all() === $_POST $request->query->all() === $_GET
$input = $form_state->isMethodType('get') ? $request->query->all() : $request->request->all(); $form_state->setUserInput($input);
$form_state->getValues() is already modified data that is formed on the basis of $form_state['input'] and #default_value
Pay attention to
#value_callback it forms the value of the field