If there's no blockage in that overflow line, which apparently not cause the bottle is filling, than as your bike cools and temperature drops, that creates a vacuum and should suck the coolant from the bottle like a straw. Back into the radiator.
If your rad cap is not holding that vacuum, then your bottle will stay full. Just like you were predicting.
Did you examine it real close? Also any hole in the overflow hose may also cause that, or hell, even loose clamps.
Also, check the nipple on the rad filler neck to make sure it's getting good flow. The one the overflow hose hooks to.
Hope that helps.