Content script is not injected for some iframes

In this site, user agent spoofing of our extension is not working.

The problem is, that the site is using an anonymous iframe element (without src or srcdoc), it seems. Content script injection (for all frames enabled) is not working in this situation. Why is this happening?

Also, Firefox automatically spoofs navigator.userAgent when we override the User-Agent request header, but not for iframes? Isn’t this a bug?

Added missing matchAboutBlank: true. But automatic user-agent spoofing is not working yet.