This is what the Preview shows / and live site too:
Expected
I expect to see the columns grid on preview/live site. Changing the block to "Latest Posts block" shows the proper grid, so this is a block styles issue with the theme for this block in particular.
Note: Not tested in atomic. Only tested in simple sites at WordPress.com, and can replicate any time the steps are followed. Not replicated on other themes I tested that have block styles (see any recommended theme in https://wordpress.com/themes).
Steps to replicate
Result
This is what the editor shows:
This is what the Preview shows / and live site too:
Expected
I expect to see the columns grid on preview/live site. Changing the block to "Latest Posts block" shows the proper grid, so this is a block styles issue with the theme for this block in particular.
Note: Not tested in atomic. Only tested in simple sites at WordPress.com, and can replicate any time the steps are followed. Not replicated on other themes I tested that have block styles (see any recommended theme in https://wordpress.com/themes).