Theory: Melon's lack of taste is down to depression.

Melon claims his being a Hybrid is why he cannot taste anything. That all his food tastes like sand.

However we never see Legoshi or Leano complain about this.

But you now who did complain about losing their sense of taste?

Riz.

After he ate Tem he lost his sense of taste due to the grief and guilt over what he did. Even when Riz convinced himself that Tem consented to it, deep down he knew he was lying to himself.

Melon hates himself for being born a Hybrid but he hates society more for seeing him as a monster. This self-loathing and lack of grounding manifests as Melon being unable to taste anything.