Here are a few bug that I’ve discovered during experiments with WP 6.1:

Themes

Reusable blocks won’t work if you try to export a theme. Just use regular blocks. Then your theme will work as intended. The theme will break apar.

JavaScript

Custom HTML blocks with JavaScript also don’t work.

However, here is a workaround: Just pick the blocks out of the reusable block. Q.E.D.