Hi Martin Hagara,
So, you want that module title to be h1 in the category blog page, but not be present in the article details page, right? Well, this is not possible unfortunately.
The category blog page is coming from Joomla core and modules assigned to this blog page will automatically inherited by the article details page of this category. As you know, these articles are not assigned to specific menu items but all assigned to the category blog menu item instead. So, module cannot be unassigned to articles details page. It can only be hidden with CSS, but it remains in the html, hence h1 is detected two times in the SEO test. I would like to request you to change your heading in the module with other/custom and then change the size as you want. Then it will not create an issue.
https://share.cleanshot.com/Ggf7qtKt
Otherwise you need to override the module output!
Best regards,