Deltas have a lot of votex flow at higher angles of attack. In effect they are practically all wintip at those angles. A sharp leading edge creates a lot of turbulent flow and it may be helping your model by controlling and stabilizing this vortex flow or the formation of separation bubbles on the upper surface at higher angles of attack.

The "problem" is that delta wings just do not work the same at higher angles as "normal" wings. The vortex flow becomes dominant. Ever see the shots of the Concorde at landing if there was fog or dust in the air? Huge curlicues of circular vortex flow over the entire span of the wing.