Why do some T-tail aircraft have a protrusion on the top front of the tail?
It's a "bullet fairing".  Sometimes you get the pressure fields of the vertical and horizontal surfaces in alignment so the lowest pressure zone of the vertical fin is merged with the lowest pressure zone of the horizontal tail right at the intersection of the surfaces.  It results in a more extreme pressure recovery aft and can cause flow separation and vibration, and sometimes shock wave effects at high speed.

The bullet effectively moves the pressure field of the vertical surface forward to bring it out of alignment with the one of the horizontal surface.  It's usually an add on resulting from interference issues that are discovered in the wind tunnel, or during flight testing.