Where...where did my spell go?

[Zorzi]

Most recently it has happened in Pharos Sirius when attempting to Miasma the crawling zombies during Siren.

Most frequently it has happened in Copperbell Mines (easy) when attempting to Miasma gigas as they run across to attack the wall.

This makes me think it's the mobs moving out of your vision range. If you start casting on a mob in front of you, and it moves behind/beside you, your spell will fail. If it's close enough to completion, an interrupt message wont show. The reason it never fails on party members is you don't have to be facing them to cast on them.