How to display a custom block in a view twig file?
You probably are confusing {{ header }} and {{ view.header }}.
The first prints the header section including the block:
views-view.html.twig
{{ header }}

The second one returns ViewExecutable::$header, an array of handler plugins you have configured in the View. You can try to extract configured string values like in the first example but in case of a block entity you probably only find the block id and you have to load and render the block yourself. It would be much easier though to print {{ header }}.