Splitting field value inside field template not working?
In a field.html.twig template {{ item.content }} is a render array. You can't use it in functions which expect a scalar like boolean, integer or string.

If you want to use field values in TWIG code better use the field object #items:

{% for item in element['#items'] %}
  {% set tags = item.value|split(' ') %}
  {{ dump(tags) }}
{% endfor %}


In a single value field you can access the value directly without a loop element['#items'].value.