Plus, against Semirhage might also speak that she apparently just got Moridin's order not to harm Rand and knew of the lesspn Mesaana got after disobeying. Besides, such an act might anger her ally Demandred.

While neither Semirhage's nor Graendal's PoV in TGS revealed it before their death, Graendal is the most culprit. This also explains why the attack wasn't very tactical executed: Graendal isn't a good General.

IMO, she also wanted to discredit Demandred by this move, since on first thought the others (especially Moridin) would suspect Demandred to pose as Sammael (who Graendal definitely knew to be dead).