It’s probably because Eevee is missing its evolutionTargetId if I have to guess.

{
  "speciesId": "ae920eed-c027-46b0-822a-32e89444a9d8",
  "species": {
    "name": "Eevee",
    "canEvolve": true,
    "evolutionTargetId": null,
    "evolutionMethod": "happiness",
    "evolutionCost": 80,
    "shinyProbability": null
  }
}