You never know honestly, they had just recently fixed an issue where ACN Book was not dropping in halatali dungeon (as well as another one but i forgot which), i mean that is a RNG and it must have been spammed enough times for someone to find that out. so there is a small possibility coil has an issue, just a matter if SE is looking into it.

However they do have different % rates for certain items, which is kind of odd. as every item should have an equal chance. It's bad enough groups have to wait 1 week for a chance at their drop. but knowing your item has the lowest chance is just depressing.