Difficulty set in the library does not apply to the chaos dungeon (in fact, entering the chaos dungeon will reset your difficulty to 0) as it uses its own difficulty system determined by depth, but when you fight old king it is essentially at difficulty 9. However, difficulty stops scaling luck at 4x which I already accounted for. If you had over the required luck then I actually do not know why you couldn't inflict break besides maybe trying to use break on a mirror that wasn't yet chanting.
Btw, depth 19 multiplies enemy stats, besides luck, by 100 while diff 9 only multiplies stats by 7 resulting in enemies at depth 19 potentially having more stats than the dlc 3 bosses despite them having a separate multiplier in addition to difficulty. Also, the boss rush only goes up to depth 10 or a 10x multiplier.