I'm here, I'm here!
Sorry it took a bit to get a response.
We received word back from the development team in regards to your question about why Pining Nocturne will not stack with other magic spells.
There have been a lot comments asking why it does not stack when Elegy and Slow, Carol and bar-spells, and Requiem and other DoT spells all stack. However, there are no set rules about what spells and song effects can and cannot be applied at the same time. Each spell and song is decided on a case-by-case basis along with their effects.
For example, the reasoning behind being able to stack Elegy and Slow is that the monster will still be able to use their special abilities so it’s forgivable.
On the other hand, with Nocturne and Addle, since the enemies casting time could be drastically increased, it would become possible to greatly exploit magic-focused enemies in battle.
While it’s fine to have somewhat of an advantage, however, while someone is casting it is not possible to perform any other actions, so prolonging this state is equivalent to making the enemy completely powerless which is why we decided to make the rule that the two spells cannot be stacked.