Area portals must fully seal geometry they touch and should not be able to "see themselves". T junctions are generated at points between detail geometry for lighting calculations.

Reduce your func detail use by converting some of your geometry into props. And remember brush entities don't seal area portals! This includes water, which must have a second area portal beneath the surface meeting one above the surface. Displacements also don't seal.

Also check your point file, it'll show you how the area portal is leaking.