Uh, since my Steam kept removing WF from my library, I'll just make this thread in case someone else would need it.
If your game disappears from the library even though the store says its already in there, and hitting the 'Play' button asks you for confirmation to install it, don't.
To fix this without re-downloading, close steam first, then you'll need to edit your WF's appmanifest file named "appmanifest_230410" at C:\Program Files (x86)\Steam\steamapps. Open it with notepad then paste the following: (Note: Don't forget replacing the "LastOwner" with your steamid64, you can get that in https://steamidfinder.com by pasting your steam profile URL in there.)
So, based on my experience, Warframe will disappear from your library if you force shut down (holding down the power button or unplugging the power) your PC with Warframe still running. Each installed game has its own appmanifest file, and apparently, WF's would magically go blank so you have to put those text up there to restore it back to your library.
(Sorry with bad english, if I had.)