This is a follow-up to #1613 specifically to @joemcgill's suggestion:

Oh, additionally, we should probably update the logic below so that if a slug is not found in the cache, it tries to request it from the API instead of returning a WP_Error right away.

When a new feature plugin is published, it may not yet exist in the perflab_plugins_info-v2 transient. Therefore, whenever one of the expected $standalone_plugins is absent, we should act as if the transient was not set in the first place so that we can obtain the latest plugin info right away.

Related: #1616