UPDATE 4-13-2021: NVIDIA has stated that the missing games have been restored to the GeForce Now Library. They are monitoring the issue to make sure it does not happen again.
It is currently unknown as to what games are missing from the GeForce Now library.
In other news, it looks like Death Stranding is now out of maintenance. GeForce Now is monitoring all in-app feedback that is provided by those who are playing the game.