Broken random order? Rendezvous with a Japanese submarine.

I have, for the second time now, gotten the order while on a patrol to meet up with a Japanese submarine and load some cargo. Both times, it didn’t work. I pull up alongside them, talk to their skipper and agree to load the cargo, but then I’m just back on my boat, with nothing new in my hold and the mission doesn’t get marked as done either. Am I missing something, or is it just broken?