If anyone is working on block-based themes and have upgraded to v9.9, you will likely experience issues, notably your theme styles not showing up in the Site Editor. Note the two PRs below. Hopefully this will save someone else the hours I spent thinking I was doing something wrong with the experimental-theme.json 😉

https://github.com/WordPress/gutenberg/pull/28731
https://github.com/WordPress/gutenberg/pull/28792

Things are getting fixed and hopefully we will have v9.9.1 soon!