Cluster rockets - cast animation not playing

[TheHook]

There is a custom spell in my current map, based on cluster rockets. It's quite a simple edit - instead of multiple rockets unit throws one rock using Art- animation "attack, defend".

It works fine at lvl 1. However, when upgraded to lvl 2 and above, the animation for casting unit stops playing - footman just stands there motionless and a rock appears. Is this problem somehow linked to engineering upgrade?