any solution or workaround?
Nope, not really. The leak is caused by AE's uniform texture filtering, which also includes edges - the layer is treated as an infinite plane, not geometry. You may try to minimize it by increasing texture map size in the advanced comp settings and overlap the layers ever so slightly, but one way or the otehr you will still see issues. You may also try to close the gaps by applying 2D treatments onto the shadow after it has been pre-composed. Using Minimax, Simple Choker or third-party plug-ins liek RevisionFX' RE:Fill you may be able to close the gap.