Why does #submit apply to all buttons?
Give your submit button a unique and valid #name attribute. I don't think it will work if you put spaces in it:
$form['actions']['save'] = [
  '#type' =&gt; 'submit',
  '#name' =&gt; 'standard-save',
  '#value' =&gt; t('Print and Mail Check'),
  '#button_type' =&gt; 'primary',
  '#submit' =&gt; [
    '_my_module_form_submit',
  ],
];

Submit handler
function _my_module_form_submit($form, \Drupal\Core\Form\FormStateInterface $form_state) {
  \Drupal::messenger()-&gt;addStatus(t('Standard Save button triggered.'));
}