Enabling this patch has brought some insight into a @todo comment in GatsbyEntityLogger.php.
// @todo Document why this is necessary for this entity type but not others.
if ($action !== 'delete') {
if ($entity->getEntityTypeId() === 'paragraph' && $this->config->get('recursive_relationships')) {
return;
} ...
With this patched option enabled, paragraphs are completely siloed from being included in fastbuilds. I believe the assumption was they'd be captured in recursive crawl of relationships of parent nodes. Which makes sense to reduce overall gatsby logs.
I think this check should be enhanced a bit to at least check the new config option is set, and maybe also that paragraphs are explicitly set as an entity to pass to build/previews. Updating that legacy @todo comment would be good too.