HI, i just tried with Gutenberg and its working fine to grab first image from content.
Please see here is my testing.
I also checked your page, can see the image is grabbing fine, but problem is image source, please see the screenshot
Can you please try with plain image url without base64 and see what’s happening.
Welcome to our forum.
Thanks for report the issue, i will take a look with Gutenberg, and let you know the update.