Should I be worried if Steam forgot I installed a game?
I imagine that the library cache file became broken you could probably rebuild it to get it to remember but steam should check the location before installing and see what is there and since your games will be all installed then it should not hurt anything. the real issue is why is the library file/s getting corrupted?
But in all you should be fine and there is nothing to worry about. It has happened on a few of my games before and all has been well.