Why exactly does a shield die instantly when it gets hit by a phantom? Shouldn't it get trapped in a subcell and get killed as if it was being attacked from 4 sides?
Shields die instantly if creeper is on a space under the shield - the shield only takes damage and blocks the creeper surrounding it, not under it.
It appears to me that Shields die in exactly the same way as any economic structure. If it has creeper on any of the 9 subcells, It dies. however, It also prevents the spreading of creeper while active, hence why it doesn't die instantly. You can get the same effect by deactivating your shield.