From pages 16-17:
“Any units aboard a transport are considered cargo until they offload. Cargo cannot take part in sea combat and is destroyed if the transport is destroyed.”
“At the end of this phase, the player controlling the corresponding territory (or the original controller if it’s contested) rolls one die for each of your ships that moved into or through each enemy minefield sea zone. Each ship is called out one at a time and one die is rolled. A roll of 1 will mean the removal of the ship, except for undamaged battleships, which instead become damaged.”
Mines are rolled, according to the rulebook, AFTER the movements. The cargo becomes cargo before the dice are rolled, and remains cargo through the mine roll if they don’t unload that turn or if the plan is to unload after clearing the SZ. Come to think of it, according to the rulebook, transports that unload without combat can still unload their cargo even if struck by a mine:
“If the sea zone from which you are launching the assault or reinforcement is already friendly, you simply offload your units into the territory being reinforced during this phase.”
The transports unloaded before mines were rolled, so there was no cargo aboard to go down with it.
I’m not trying to give you a hard time, I’m just trying to point out that mines are pretty unclear.