This has happened to me now in 2 campaigns.
I have a nice Shimazu campaign going on Hard setting. It's the spring of 1565, and I have all of Kyushu, and the 2 westernmost provinces on the main island. Building everything up nicely. I've got 5 trade ships on all 3 western nodes, the one in the south protected by 4 medium bunes. All of a sudden those 9 ships on the southern node are bounced off of it by ONE Yamano trade ship. He just comes to it and displaces the entire fleet!?!! Of course I can't take it back without going to war with them, which I'd prefer not to do at this point.
Has this happened to anyone else? Am I missing something?