Custom styles of core/group invisible in editor

[aartjan]

Describe the bug
Before activating the Gutenberg plugin (working in WordPress 5.3.2) I could use JS or PHP to add a custom style to the core/group block and the backend would provide a "is-style-XXX" class to the group. This is still working in the website, but not in the editor when I activate the Gutenberg Plugin.

To reproduce
Steps to reproduce the behavior:
Add this JS:
registerBlockStyle( 'core/group' , { name: 'golden-border', label: __( 'Golden Border' ), });
or this PHP:
register_block_style( 'core/group', [ 'name' => 'golden-border', 'label' => 'Golden Border', 'style_handle' => 'block-styles-stylesheet', ] );
to a theme, without installing Gutenberg Plugin.
Add this to the stylesheet of the theme
.wp-block-group.is-style-golden-border { border: 2px solid #caa071; (...) }
Add this to the theme:
add_theme_support( 'editor-styles' );

Expected behavior
Both in the website and in the editor a 2px border should appear around the group if I select the style for a group. This happens if I disable the plugin, but with the plugin enabled, the is-style-golden-border class is gone.

Screenshots
Fragments of editor with Gutenberg plugin disabled

Fragments of editor with Gutenberg plugin enabled

Fragment of the website (identical in both cases)

Desktop (please complete the following information):

• OS: OSX Catalina
• Browser Chrome
• Version 79

Additional context

• Gutenberg plugin 7.4.0

[talldan]

@aartjan Thanks for reporting the issue. I can confirm the bug. Looks like it's caused by this change:
https://github.com/WordPress/gutenberg/pull/19181/files#diff-d819635dc4c6f7b9326773f5726a37b7R36

I'll work on a fix for this.