Dynamic blocks no longer rendering with their wrapper classnames on the frontend

[talldan]

Describe the bug
Since #26356 was merged, dynamic blocks no longer render with the wp-block-blockname class on their wrappers

To reproduce
Steps to reproduce the behavior:

1. Add a dynamic block (like Search)
2. Preview the post
3. Inspect the block and notice that the wp-block-search classname is missing.
4. Some blocks may not be styled correctly due to the missing classname

Expected behavior
The wp-block-search classname is retained.

Screenshots

Search block with the wrapper class

Search block without the wrapper class

[talldan]

This is possibly fixed by WordPress/wordpress-develop#640, but I'm not completely sure.

[talldan]

Just to confirm why it was closed, this is fixed by WordPress/wordpress-develop#640.

[oandregal]

@tellthemachines for awareness.

As far as my testing goes, this error was only reproducible with WordPress 5.6 beta1 + Gutenberg master. The variations I've tested are:

WP 5.6 beta1, no gutenberg plugin => fine.
WP 5.6 beta1, gutenberg 9.2.1 => fine.
WP 5.6 beta1, gutenberg master => no class attached for dynamic blocks + PHP errors.

WP 5.6 beta1 + WordPress/wordpress-develop#640, no gutenberg => fine.
WP 5.6 beta1 + WordPress/wordpress-develop#640, gutenberg 9.2.1 => fine.
WP 5.6 beta1 + WordPress/wordpress-develop#640, gutenberg master => fine.

WP 5.5, no gutenberg => fine.
WP 5.5, gutenberg 9.2.1 => fine.
WP 5.5, gutenberg master => fine.

[oandregal]

For full context: it turns out that in the widget screen (preview widgets) the issue can be reproduced with 5.5.1 and Gutenberg 9.2.1, see #26420 For this to be fixed it needs #26417

[tellthemachines]

Thanks, @nosolosw !