Historically the Curse did not affect humans that had mixed ancestry (i.e. Lutin, Elf, ect).
Any non-human constituent in the blood was enough to prevent the curse affixing itself to mixed-bloods though it did catch a few by some unknown means (a mixed human/lutin was affected, as was a Kitsune).
The Curse is strange and fickle… that's the hard and fast rule you should consider. It just does not act as one expects that it should in particularly strange situations.
And those anti-curse amulets/charms/tattoos/implants are only expected to hold it at bay under normal circumstances…. *evil laughter*