So this is one from a Malz main. The rune clearly states "damaging a champion below 50% hp" causes it to proc. I can therefore only assume that Malz voidlings and Liandrys Torment should proc it, and them failing to do so is a bug?
Also I noticed a while ago malz voidlings don't proc liandrys torment despite a similar condition of "spells" which his voidlings ARE. It works for works for Yorick ghouls, Heimer turrets and Zyra plants so not sure why the voidlings are the exception unless this too is a bug? If so it is a VERY long standing one (note Rylais works just fine on them).