Very glad to see that ACF almost ready with Gutenberg!
I have seen there’re a problem when you change the block in the code editor mode or deactivated the Gutenberg Plugin, the block code doesn’t render in HTML properly.
Thanks for the email,
Elliot here – ACF dev.
ACF Blocks are dynamic, meaning the “HTML” is left empty, and all data is saved as an object within the “HTML comment”.
ACF then renders the HTML via your “render_callback” usng this data. I hope this info is helpful.