Some Block Bindings are only editable if provided with a default

[bacoords]

FYI - This may be a bug we report upstream to Gutenberg, but with the Block Bindings API, you can't use an empty attribute in the template. All fields have to have some content initially to be editable. So for example, if you don't add alt text to your image block in the template, the alt text isn't editable in the single post view. If you don't have a link destination in a button block, the link destination is not editable.

We can seed some default content, but it seems a larger issue, like when you want to just show placeholders or when you just want the data to be empty!