@perrelet can you take a look at the new documentation here at https://www.advancedcustomfields.com/resources/whats-new-with-acf-blocks-in-acf-6/ to see if it helps.
There’s a quote that may or may not be applicable to your question:
WordPress 5.9 users who use Block Styles may have noticed that the ACF registration of styles was not applied to those previews, which is why all ACF Block users should migrate their blocks to use those WordPress provided methods of asset loading.
