Reported at p1HpG7-64l-p2 #comment-29600:
No matter what settings I set, I could not get List View to work. Always showed my posts in Grid View after publishing. Same results in Preview.
The block to be rendered in list mode.
The block was rendered in grid mode.

I've verified that the correct class is applied on the frontend (.jp-relatedposts-list). I think the stylesheet's selectors aren't working since they require an element with ID #jp-relatedposts wrapped around Related Posts. (ID based styling probably isn't a good idea here -- or anywhere really -- anyway.)
Seems like we don't handle lists on WP.com the same way we do for Jetpack. Will work on a fix.
Fixed by:
Both of the above have been merged, and on production things are now looking well. Closing.
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Fixed by: