At first I thought that Halsin offers to join in drow scene even if you reject him because of some missing flag, game not recognising that you are not in relationship with him, it would be pretty common for act 3, for example I would get Gale talking about deal with Raphael before even meeting him in Sharess' Caress. So I did a little research (I promise it was for science) and watched on youtube every companion reaction to poly and then their reactions to drow offer. And guess what? There is no way to be in relation with both Gale and Halsin, because Gale refuses to do poly, but Halsin will still ask to join in drow scene. So him asking to join with PC and Shadowheart even after you reject him is almost for sure intended and not bugged, he just cannot stop himself when he see those twins. So I don't know about you guys, but for me SH/Halsin is canon
