We will need to look at this in more detail because there is certainly a bug here, but I do at least have a workaround for you: the reason it’s going bananas is because the piccolo is assigned to only one flow, ‘Der Dandy’. Before you try to drag the piccolo instrument to the Flute player, first select it in the Players panel, then switch on the checkboxes for each of the other three movements, so that the Piccolo player is assigned to all four movements. Then you can drag the piccolo instrument to the Flute player, and it should work. Make sure that both the source and destination players are assigned to the same flows before you attempt to move the instruments, and you should be OK.