Is it possible to make manual teaser breaks work with "Filtered HTML"?
This comes down to being a problem of DomDocument and its childNodes property that does not include DomComments. Specifically, Drupal\filter\Plugin\Filter\FilterHtml::getHtmlRestrictions() will not consider any comment-like tags that are added in the 'allowed HTML tags' list. This is due to these lines in that function's code:

$html = str_replace('&gt;', ' /&gt;', $this-&gt;settings['allowed_html']);
// Protect any trailing * characters in attribute names, since DomDocument
// strips them as invalid.
$star_protector = '__zqh6vxfbk3cg__';
$html = str_replace('*', $star_protector, $html);
$body_child_nodes = Html::load($html)-&gt;getElementsByTagName('body')-&gt;item(0)-&gt;childNodes;


This loads the text of the 'allowed HTML tags' text as an HTML DomDocument and retrieves the list of the children. Only 'proper' HTML tags will be listed as part of the childNodes property and therefore, the Drupal 7 workaround will not work for Drupal 8.

In order to solve this, one could override the FilterHtml plugin in order to allow &lt;!--break--&gt; to remain in the text.

One attempt following this approach exists here:

https://www.drupal.org/project/drupal8breaks

which was in turn inspired by

https://medium.com/@djphenaproxima/how-to-bend-drupal-8-plugins-to-your-nefarious-will-94da0c31f095

Good luck!

P.S.: Also available on GitHub https://github.com/stefanospetrakis/drupal8breaks