The spell ends on B's first turn, as that's their "next turn", which means yes, timing matters. While it's often better to unload a spell before an enemy's turn, many debuffs are best after instead. If you think of the spell (or Demoralize, etc.) as setting up for your allies, then it makes a little more sense that you want to go before them rather than before the enemy.
If it seems a tad unfair, note that the creature did succeed, so it was a minor effect, a consolation prize of sorts.
This timing is one reason to separate monster initiatives, so they too don't always have perfect timing by being clumped, at least not without Delaying to act so tactically.
This is why delaying your intiative in order to cast your spell *after* an enemy acts can be a viable choice.
Or not use a spell that is gimped by both the incap trait as well as a rather bad sucess effect.
