Item template inherits variable from blog template, even if it is defined to false


In blog template and item template I have the sidebar variable in this way (same as the Quark theme):

{% set show_sidebar = header_var('show_sidebar', [page, blog])|defined(false)  %}

If I define that variable in the blog as true, the child pages (item type) inherit that value, although on these pages the filter |defined is set to false.

If I do not define anything, by default, the sidebar will be displayed on the blog page and on the item pages, it will not be displayed, that is, the |defined filter does have an influence.

It is very annoying to have to set the show_sidebar variable to false, explicitly in each blog article.
Is there a way to avoid this inheritance between item type pages with respect to their parent blog?