Here is the in game perk description for context:
“At random times during battle, your creatures have a 0% (+1% per rank) chance to cast a random, Ethereal Spell Gem from their spell list.”
Based on the way it was written and its effect in battle, Spontaneity seemed like it was supposed to be one of those low cost scaling perks you had to invest in multiple times for a significant effect. In game, it has only 1 rank, capping at 1%. I wasn’t sure if this was intended or not, so I figured I should post here just in case.