In prior patches, the developers have changed the tomestone reward accompanying the removal of the lockout for each set of Coils from rewarding only the limited currency to a mix of limited and unlimited currency.
  • BCOB: 50 mythology (2.0) --> 20 mythology, 10 soldiery (2.2)
  • SCOB: 30 soldiery (2.2) --> 20 soldiery, 10 poetics (2.4)

Since a new currency was not introduced with the unlocking of FCOB, I do not think it's necessary to follow this pattern exactly to the letter. However, it would be nice to earn a little bit of soldiery while farming FCOB for loot every week, instead of just capping out poetics immediately and then getting the error message after every subsequent run.