When sprites mix across species, the child takes on more traits of their biological mother because of birthing constraints and the hormones available. Without new mixed genetics, it takes about four or five generations for mixed traits to disappear, however the connection to the Spirit takes almost three times that long to completely fade.
When mixed, the Spirits share a connection to the sprite. The resulting sprite may Pray to either, but can never have the full connection required for the Voice of the Spirit. Because of that shared connection, the Spirits involved must get along well enough and trust one another to be so close to a potential flow of their magic. Some sprite pairings cannot produce offspring because of this.