"Edit as HTML" breaks Cwicly Blocks

When I try to edit Cwicly blocks as HTML, I get an error saying “This block contains unexpected or invalid content.”

Here’s a screenshot showing a comparison between Gutenberg blocks (paragraph & heading) with corresponding Cwicly blocks (paragraph & heading).


Both sets of blocks have the same content, and both are set to Edit as HTML. The core blocks work okay, but Cwicly’s return an error.