When including a non-existing section from a page (or namespace), the include plugin includes the complete page. Instead, it shouldn't include anything (possibly with a configurable (warning) message instead).
There is a partial workaround (https://www.dokuwiki.org/plugin:include#how_to_avoid_inclusion_of_whole_page_content_if_section_does_not_exist), but that's not always a doable solution. Esp. not if one imports sections from many files, which need to result in a proper "presentable" summary page. See also https://www.dokuwiki.org/plugin:include#empty_section