For now, I can only assume that the constant casting of Slow is just a lucky random occurrence (possibly in combination with the fact that other frequently cast spells are already applied to all stacks). Although, in theory, the random element shouldn't repeat in offline mode.
The Wizard might also not cast any spells if all possible spells are already applied. In theory, there shouldn't be any other reasons.
If the save file can be found (along with the necessary sequence of actions), or if the situation can be recreated, I will definitely take a look and try to figure it out.