54 Games Missing From Steam Library

In all probability a specific filter is set in the new UI, preventing you from seeing all games.

There are several ways this can be changed:

1. Maybe the dedicated button to filter out games that are currently unavailable is switched on:

   

2. There is a myriad of preset tags to choose from to filter your game library to create 'Dynamic Collections', and this might have filtered out part of your library:

   

3. Lastly, there is a drop-down menu to the left of the former two options, that allows you to filter software, videos, and tools. The option for 'Games' might be deselected:

   

I found the most satisfactory solution possible by experimentation:

At first, I was unable to modify or edit the categories and the items within them, because the new system works differently from the old system. The total number of games in all owned accounts are somewhat accurately tracked (accurate for currently logged-in account, not so much for the others).

Uninstalling Steam and cleaning the registry doesn't work; undesired settings are restored from the cloud upon reinstallation. The best I could do was delete all but the uncategorized groupings and/or the items within them [via context menu] (deleting all the items in a category removes that category).

I've been a Steam member since 2007 and a computer user since 1984. I had to make handwritten lists of owned games in the accounts' profiles and compare them to all the uncategorized games (a must!) before I could determine whether or not any were indeed missing. This was a process requiring many hours of boringly repetitive work, which I would not have had to perform otherwise (So thank you, Steam, for making this SO accursedly confusing).

I found for me that the "missing games" weren't actually games. I have my library set up into 2 Collections: Favorites and Uncategorized. You can see that there is a lot missing from Uncategorized:

Similar to the answer from @Joachim, if you go into the filter settings and select Unplayed, a new category called "Tools" will appear. Here I see a bunch of dedicated servers for games I've never installed, as well as a few SDK's and other things.