Can't trigger Lucanis' quest Inner Demons.

I think this is actually expected behaviour; if you let Treviso get blighted, then Lucanis doesn't do the introspection that leads to Inner Demons. He can still become a Hero of the Veilguard, it just happens later in the questlines.

Source: I played through once saving Minrathous and a second time saving Treviso. When saving Minrathous, I didn't get Inner Demons, but he did become a Hero.