|slice filter does not work with special characters
Actually the culprit is |striptags 


  The problem with using twig's |striptags is double encoding of html entities,
  not markup, so &amp; becomes &amp; and then &amp;amp; – Berdir
  
  source: How do I get the raw field value in a twig template?


If your setup relies on you running striptags last, you will need to do this all of this magic in a preprocess function with php setting a variable here and then printing your formatted variable on twig.