Rocks'n'Diamonds

When I played GavinD's Contribution 2006 levelset a strange bug occured in level 48:

Some dragons didn't stop spitting fire, they were frozen at their positions.

Here's the level and my tape: FrozenDragon.zip

I agree, that's strange!
But:
Have you seen? One of the dragons which
spat fire at this moment was killed by an
exploding robot.
I think the fire is signaled by the dragon
when it's allowed to stop burning. But the other
three dragons.....?
Hmmmm...