FAPI #prefix YAML filter tag button_submit:
   '#prefix': '&lt;div class="col-md-4 text-right"&gt;text&lt;button&gt;button text&lt;/button&gt;'
   '#suffix': '&lt;/div&gt;'
   '#value': Send
   '#type': submit


I have the code above in YAML form. When I edit the code, I see the button tag; when rendering, only the following markup is left.

&lt;div class="col-md-4 text-right"&gt;
ALGOasd
&lt;input id="edit-button-submit" class="button js-form-submit form-submit" type="submit" value="Send" name="op" data-drupal-selector="edit-button-submit"&gt;
&lt;/div&gt;


It´s clear that I have some filter, but I can't find it out. I also tried putting a &lt;button&gt; in a container item, but the same thing happens.

Why does this happen? How can I fix it?
You need to include the #allowed_tags property which is applied to #markup, #prefix, and #suffix. (See: Pass raw html to #markup)

button_submit:
  '#prefix': '&lt;div class="col-md-4 text-right"&gt;text&lt;button&gt;button text&lt;/button&gt;'
  '#suffix': '&lt;/div&gt;'
  '#value': Send
  '#type': submit
  '#allowed_tags':
    - div
    - button