From looking at the code, probably terribly unlucky, but I might need to test a save file to make sure.
For how the item drop system works, after combat finishes and it is determined as a victory and is not a game-over, potential drops will be rewarded. Note that some combat encounters may abort combat entirely (such as start combat initially, but end it after some transitional event, like moving to sex before or during a fight)--on Mhen'ga, the female zil does this depending on the options chosen--these kinds of combat endings will not count as a victory. During the rewards checking, the player will be granted any enemy-tied loot, as well as credits and XP. Additional to that, there will be a roll for rare drops and a roll for any holiday-specific drops, each tied to their own rarities.
For the rare drops, there is a 2:100 (or 1:50) chance to get a potential item, with the chances increased to 4:100 (or 1:25) chance when the "Cupid's Kiss" status effect is active (technically it's by 3x, but randomization counting is offset by one, so the chances reflect that). For Mhen'ga, there are four potential books, with two having prerequisites--so two possible books if going in fresh. The book "BXBI: I" and "Satyr Slave" have no prerequisites, and "BXBI: II" will require already unlocking "BXBI: I", and "Tentacles & Tanukis" will require having met Kiro and Syri at some point. If all the books have been unlocked, you'll start getting silver piercing drops.
Hope this helps.
