Looking through the 4.2 patch notes...
Spell Interrupts:
When interrupted while casting a single-school spell such as Mind Flay or Frostbolt, players will now be able to cast dual-school spells such as Mind Spike (Frost and Shadow schools) or Frostfire Bolt (Frost and Fire schools). However, being interrupted while casting a dual-school spell will still interrupt all respective schools as intended.
What is their reason behind this? Apologies if this is a double post or old news.