This is happening in the three realms I play on where I've been crafting books using an enchantment table or catching them via fishing.

In the one realm I've been creating and collecting the greatest number of books recently, I have exactly 100 books. Most of those have been made with an enchanting table (I always make Level 3 books), with maybe fewer than 10 being acquired through fishing, currently.

Of those 100, 25 have three spells on them, and every single one of those books has Feather Falling IV on it. A further 9 books have 4 spells on them, and every one of those books has Feather Falling IV on it as well as Blast Protection IV.

The only part of the process for 3- and 4-spell books that is random is the 3rd and 4th spells, respectively The first one or two slots appear to have Feather Falling IV as mandatory, as well as Blast Protection IV.

Making matters worse is the fact that Fortune III and Unbreaking III are not generating at all via the enchanting table. The only way I've been able to get Fortune III on my tools is to combine rare instances of Fortune II books together to make a III. The only way I've been able to put Unbreaking III on my gear is with books I've caught via fishing.