The customer observed an inconsistency when custom block styles are in use. If you select them you are not able to switch to the built-in ones e.g. Heading 1.

Steps to reproduce:
1. Add two block styles (in both cases please mark inline: false), please follow the corresponding doc page.
2. Create a new article.
3. In the Rich Text field change style to highlighted_word or highlighted_block.
4. Try to switch to one of the built-in styles like Heading 1 or Formatted.

Result:
You are unable to switch to any of the built-in styles, only highlighted_word and highlighted_block are visible in the dropdown. If you decide to use e.g. Formatted you are easily able to switch to the other default style e.g. Heading 1.

Note:
The only way to disable the selected custom block style is by clicking it again in the dropdown which isn't really convenient.