Plunder screen keeps alive until the last Convoy in the server is dispatched since the first versions of the game and not a bug as in one of my previous CS feedback which makes sense. In my old servers we used to do it in order to conceal our convoys as you did and some random strong guys just realized that you can access Plunder Screen until the last convoy is dispatched. Even some of them just started to check plunder screen between 14.00-14.30 just because of that knowing if they can access there after 14.00, there would definitely be a convoy in next 30 mins, most likely in 10 mins.