URQL: Union type field in an un-keyable entity gets wrong typename from the union

Question

So I have this fragment

fragment fullOption on Option {
  selectId
  type
  options {
    ... on NumberArray {
      values
    }
    ... on CustomNumberArray {
      id
      name
      values
    }
  }
}

All of the Option, NumberArray and CustomNumberArray are un-keyable entities.

Invalidating the Option field is obviously overcoming this issue in the context of Mutations, but I have this problem occurring with the Queries where even though the API data has the correct typenames for the respective options, the URQL Cache client messes that data and there is some option that is NumberArray but it gets treated as CustomNumberArray.

Has anyone met with a similar issue?

Tried adding __typename in the fragment definitions, and resolver __typename enforcing is also "illegal", so I wasn't sure about any other possible solutions.