It (at least sounds like) a known bug. If you have a fleet on one of those trade spots, and split it by moving some of the ships off*, it buggers up the fleet left on the trade spot fleet. You can avoid it by moving the whole fleet off the spot, splitting it, and then moving some ships back, but afaik there's no way of fixing it once it's happened.

*Not sure if this applies for all fleets or just if you've got a mix of trade/non trade ships and you try and move the non-trade ships off.