Sometimes the last spell in a multicast chain does has no effect or affects only 1 enemy. I think it has something to do with Imp Sacrificer’s ability.
It just happened with the Imp dead.
This is still happening. When a creature casts more than 3 spells in a row, the last one doesn’t hit anyone or just one creature, and if it does hit that creature, its additional effects like burn or speed down don’t trigger.
I noticed this with buffs yesterday and forgot to report it. Mine was with Truelight. If I got a cast twice temp gem on it, it would cast once fine, then the 2nd cast would hit all of the enemy party but only buff one or two of my guys before it just fizzled.