Man, the concept of Chiss Navigators "losing" their Force powers is such an underrated and tragic plot point in the Thrawn books.
Disclaimer: I haven't read the Ascendancy books yet, so this is me speculating on a topic on which all the information I possess is based on snippets of the books I've read here and there, Wikipedia pages and A LOT of fanfiction I've consumed over the years.... But bear with me. I have Thoughts™ about this.
I don't think the books ever explicitly explain why the Chiss "sky-walkers" lose their Sight, so this might be headcanon on my part, but from what I understand there's enough evidence sprinkled in about the fact that it might be linked, at least to some degree, to the trauma and stress Force-sensitive children are put under in Chiss society. It is well established in Star Wars lore that Force powers can't just "disappear", so unless Zahn is messing with established Force lore, it cannot be interpreted as a natural occurrence linked to, idk, a quirk of Chiss biology.
I think this speaks loudly about the underlying dysfunctionality of Chiss society. From what I can gather, it seems to me like a system that demands the complete subjugation of the individual to achieve a collective goal... or at least, that's how it's sold to its citizens to keep them compliant. In reality, we readers know it is all in service of a select few (the heads of the Ruling Families). Man, the Chiss are such a dark mirror to the Empire, when you think about it.
Anyway, I think that the fading of Force powers in "sky-walkers" HAS to be linked with the inherent dysfunctionality of Chiss society. It's probably a (subconscious) reaction to the brainwashing and forced compliance to a fundamentally selfish system, which is purely aimed at ensuring the dominance of the Ascendancy over the rest of the Chaos. I think it is deeply symbolic that Vah'nya, the only Sky-walker whose powers don't fade, is one of the few who seems genuinely content in her role among the CEDF. For her, it was a choice to serve, so this attunedness helps her preserve her connection with the Force. I think we are meant to understand that for other Chiss girls it wasn't so, and that this led to a sort of subconscious suppression of their Sight, which man. It's so tragic. I can easily imagine how they would eventually become resentful towards the power that basically ensured their enslavement.
I think it's also possible that, since Chiss culture lacks the mystical tradition that appears to be foundational in order to properly nurture and develop Force powers among Force user communities, this would probably accelerate the suppression process. The navigators really have no support whatsoever.
It's also very interesting how this all relates to Thrawn. I've always thought there's a contradictory quality to his character when it comes to his attitude to the Force: on one hand, his utilitarian and analytical attitude causes him to be... almost in denial of the Force's capabilities. Yet, at the same time, his love and understanding of art means that, on an instinctual level, he's also deeply attuned to things that are impalpable, whimsical, mystical and symbolic. It's almost like there's a deep cognitive dissonance in Thrawn himself, which prevents him from seeing the full picture of what the Force is and how it actually works.
Man, they are all so trapped in a prison of their own making, and they can't even see it due to their own cultural biases. ç_ç I just want to hug them all ç_____ç
