Take the coil out and see if the black in color o-ring/gasket in the coils thread is present, and/ or seated properly. Overfilling the tank can cause juice to run down the tanks center post too, and then reach the battery poles (wet poles).
Also, the transparent silicone sleeve on the coil, which seales the coil and tank with the mouth piece is sometimes not pushed down far enough, or cut too short from the factory, so that the silicone sleeve can't seal with the wicks a little, which can cause light leaks too. If the wicks don't get slightly squeezed together and pushed against the coil housing and the silicone sleeve, then a little bit of juice can leak from the tank into the center post/air pipe. Or in other words, some juice bypasses the wicks due to the play the wicks have in the coil body. But if the wicks get too much squeezed, then your wicks might not get resupplied with juice fast enough to rewick. So the right amount of "wick squezze" is needed, lol. Hope everything makes sense. My English still needs some improvement.