Vanquisher's Book of Knowledge appears to not drop if done out of order.

I just did 2 T15 irradiated boss maps in a row - The first one dropped Vanquisher's book of knowledge I (This seems normal, I could've done this on a regular T15)

Upon beating the 2nd T15 irradiated map boss, Vanquisher's book of knowledge II did NOT drop.

Expected behaviour is that book 2 should have dropped from the 2nd boss irradiated map. (Level 80)