The issue comes from how the data is structured. The structure served by the Classic block breaks our ability to parse other blocks on the page. We’re looking into how to fix it, but no luck as of yet.
A plugin like this may be helpful, but still requires you to at least load the post and save it: https://wordpress.org/plugins/convert-to-blocks/
There’s a bulk converter, but it seems it’s been abandoned, unfortunately: https://wordpress.org/plugins/bulk-block-converter/
I’ll keep this open so we can suggest a fix once we find one. For now, the only solution is to get those posts converted, unfortunately. I’m sorry I can’t be more helpful!