Alas, you didn’t paste in the line that opens the VirtualHost configuration block and indicates which virtual host it is. In any case, it is probably explicitly tied to port 80 or is the first VirtualHost configuration block reached in your configuration and is therefore the default.
Somewhere in your config files there should be a VirtualHost configuration block controlling what listens on port 443. Try putting a copy of the RequestHeader command there.
That said, Apache config files can be sliced and diced and included and setup many, many different ways, so what I say may not be completely true for your configuration. But what is probably happening, is that the required header is only being added to requests that originally arrive on HTTP and are then rewritten. It is not being added on requests that originally arrive on HTTPS.