Custom Gutenberg Block adding in IFRAME instead of string containing URL

I am trying to create a simple custom ‘testimonial’ block. I have been following the following tutorial

https://themes.artbees.net/blog/create-a-custom-block-for-wordpress-gutenberg/

and created a block containing two <p> elements to hold testimonial content, and testimonial author information.

The testimonial author field should contain a URL, and everything works fine in the editor. But when I review the page, some but not all URL’s are displaying fine as text.

A few, however, are not showing at all, and WordPress has added a <blockquote> and <iframe> pulling data from the URL instead.

I have not been able to find any help in the WordPress Block API reference.

My testimonial.js is as follows: