View is removing iframe from custom text
This is not the expected behavior of drupal views, Drupal Views usually not doing this.
I have done this before, and global text with iframe is working with no issue.
Maybe you have another issue like some javascript issue to prevent the load iframe form another site, see the console of your webpage maybe you got some error.

You can debug also by putting that iframe into any page Textarea field where you already use the HTML content of your website.