You sure you don't have Civ 6 hidden? If it's hidden, a game won't show in your library.
But this is one of the dangers of an all-digital future. Companies are the ones in charge of your products rather than yourself. Steam is getting ahead of a new law that requires digital storefronts to display that you're only purchasing a license of the game. Not the game itself.
Only proper digital store that 100% lets you OWN your digital games is GOG.
You called down the thunder, now reap the whirlwind