"Standalone Expansions" do not display links to the base game.
Currently, if a title is stated to be an Expansion for another game, the Expansion appears in the original game's page. However, Standalone Expansions do not get the same connection, either in the base game or in their own page, even though the option to link them is in the Edit page of the Standalone Expansion.
Examples: MechWarrior 2 and Mercenaries, Uncharted 4 and Lost Legacy, Spider-Man PS4 and Miles Morales
I believe this is now solved. Standalone Expansions will display on the Related Content, and on the Standalone Expansion it will mention the parent game.