Album posts from Social appear as a single post of the album on Facebook when three or more photos are included in the album. In an album post which contains only two photos, the post always appears as two separate posts.
The split of the album into two posts is caused by a Facebook API limitation. Facebook does not provide an API to post an album. Social sends the individual photos and Facebook generates an auto-post (an aggregated album post) from the photos to create the album and aggregated album post.
Social moderation pulls the individual photo posts back into the system. In analytics reports, they appear as individual photos.
In summary, a Facebook album post of only two photos from Social never aggregates as an album on Facebook. You must have three or more photos in the Social post for it to work. Sometimes, Facebook may still break an intended album of three or more photos into separate posts by failing to aggregate them, but it is less likely.