Board milestone has wrong ID

When fetching a board in GraphQL

query GroupBoard($fullPath: ID!, $boardId: ID!) {
  workspace: group(fullPath: $fullPath) {
    board(id: $boardId) {
      id
      name
      hideBacklogList
      hideClosedList
      milestone {
        id
        title
      }
    }
  }
}

the milestone id returned for Any milestone title is gid://gitlab/Timebox::TimeboxStruct/-1 instead of gid://gitlab/Milestone/-1.

Identically, the iteration ID is incorrect and the following gets returned:

"iteration": {
  "__typename": "Iteration",
  "id": "gid://gitlab/Timebox::TimeboxStruct/-4",
  "title": "Current"
},

Related to #341331 (closed).