Posted January 01, 2018
You're right. Strange, this usually happens only with games that normally cost money and are only temporarily free like Kingdom: Classic was a few weeks ago. My guess as to the reasons most F2P games disappear from the library are:
BeatriceElysia: Try Heroine's Quest: Herald of Ragnarok - excellent game and free. There's paid dlc, but there's no need to buy that.
Hirkis: Not to mention it can be added (+1) permanently to your library, if you care about such thing, unlike most other free-to-play games which don't count towards your library count and can only be installed and played.
1. to not clutter people's libraries with games they tried and stopped playing.
2. because number of owned games is directly connected to the Game Collector badge progress and if every F2P counted towards that then the lowest ranks would have been worthless as everybody would soon be at least a "Power Player" which requires owning at least a 100 games.
My guess is Heroine's Quest was not properly entered into Steam as a Free To Play game but instead as a normal game with a price of zero, the reason I think this might be the case is because AX:EL - Air XenoDawn which previously cost money is now F2P and remains in the library even if you try installing now for the first time.
If I find any other steam games that behave like this then I'll be sure to report it here.