After upgrading to WordPress 5.3 (no changes to my metabox blocks), none of my metabox blocks show up in Gutenberg. When opening a page that already has metabox blocks on it, it shows instead of the block: Your site doesn't include support for the "meta-box/twt-featured-text-block" block. You can leave this block intact or remove it entirely. On the frontend, the blocks are still rendered correctly though.
When looking into the console, it shows the following error:
Uncaught TypeError: Cannot read property 'extend' of undefined
at Module.<anonymous> (blocks.min.js:1)
at n (blocks.min.js:1)
Here is the code in blocks.min.js that throws the error: