I've always wondered this. I'll try to explain this by giving an example...

Player 1 fleetsaves to planet X with an attack mission carrying 1,000,000 metal.

Player 2 catches player 1 on lanx and sets up an ACS Defend mission on planet X.

The ACS defense works and player 1's fleet is destroyed. What happens to the carried metal? Is it left in the DF or does it just disappear?

it disappears unfortunately.

I've always wondered this too. Thanks schnitter.

Same with any ACS defense mission that is carrying resources when destroyed.

Well thats no fun.