Ok, with a complete fresh install of everything now it’s working with the lates beta2.
No, I didn’t. Stackable Blocks for Gutenberg work with newer nightly builds of wordpress. But I still have problems with the AFC blocks.
Ok, I think this is also a wordpress beta problem. I also have problems with Stackable Blocks for Gutenberg, too.
There’s a diff here: https://core.trac.wordpress.org/ticket/45172
This works for me. I can see the metaboxes with custom fields now.