[thejeff]

As I suggested above, this was most likely a case of the AI deciding not to waste gems on a weak enemy.
This is usually a good thing. It prevents a huge late game army from mindlessly following its script and burning through dozens of gems when attacked by a single scout. Especially important when you're storming a castle later that turn.

It's very rare these days, for the AI to override a script if the spell could be cast effectively. Not using gems is the main exception. Otherwise it will usually only override if there isn't a valid target in range.
To use Gandalf's terminology, if the pretender said "cast Frozen Heart", I've never seen that overridden by line of sight or unit type etc. The spell has a range limit. If there is an enemy unit who isn't 100% CR within range, the mage will follow the order.

There used to be a few spells that were bugged and would get priority over even scripted spells, Astral Shield and Arrow Fend, possibly others. Those have been fixed and I haven't seen reports of similar problems in awhile.