Tested with Counter and Articles blocks, so I assume this happens for all blocks that we converted to WYSIWYG... It's possible to edit them as HTML, but then when leaving the block or trying to switch back to visual editing, it shows "This block contains unexpected or invalid content."

From there, the user can either:

• "convert to HTML" (which is not reversible, and doesn't show the block preview)
• "attempt block recovery" which does bring the visual editor and preview back, but changes made in the HTML are not saved.

Update
Ticket re-purposed to match the decided solution. For most of our WYSIWYG blocks editing in HTML doesn't make sense. Some are dynamic (eg. Articles) and other are complex enough to make them prone to errors if HTML Edit is used. Some of the Gutenberg native blocks also have this feature disabled.