If a client provides an invalid format when creating or updating a Post, the current behavior is to:

1. Create the post with all other supplied details.
2. Silently fail setting the post format (we create a WP_Error but don't do anything with it).

See https://github.com/WP-API/WP-API/blob/develop/lib/class-wp-json-posts-controller.php#L156

As a client developer, I'd expect my data to be validated before any data transformation happens, so I don't have to worry about my Resource only partially being updated.

This applies more broadly to other attributes too, not just post formats.