Hi,
I don’t know if the template behavior is intentional, but you’re right. As soon as you create a template (whether header, single, or other) and assign it to a page by condition, the template overwrites everything that you previously created in a page in the Bricks Editor.
“Post content” shows absolutely nothing on the bricks side. If the content was created in Gutenberg, it will be displayed.
This can actually not be wanted (especially not if it is a header or footer template). Or we completely misunderstand the template system … 