Monzo will post a technical write-up soon. But, in the meantime:

Before:

  .method public abstract pan(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Lio/reactivex/Single;
      .param p1    # Ljava/lang/String;
    .annotation runtime Lretrofit2/http/Query;
        value = "account_id"
    .end annotation
      .end param
      .param p2    # Ljava/lang/String;
    .annotation runtime Lretrofit2/http/Query;
        value = "card_id"
    .end annotation
      .end param
      .param p3    # Ljava/lang/String;
    .annotation runtime Lretrofit2/http/Query;
        value = "challenge_type"
    .end annotation
      .end param
      .param p4    # Ljava/lang/String;
    .annotation runtime Lretrofit2/http/Query;
        value = "challenge"
    .end annotation
      .end param
      .param p5    # Ljava/lang/String;
    .annotation runtime Lretrofit2/http/Query;
        value = "idempotency_key"
    .end annotation
      .end param
      .annotation system Ldalvik/annotation/Signature;
    value = {
        "(",
        "Ljava/lang/String;",
        "Ljava/lang/String;",
        "Ljava/lang/String;",
        "Ljava/lang/String;",
        "Ljava/lang/String;",
        ")",
        "Lio/reactivex/Single<",
        "Lcom/monzo/card/data/api/PanResponse;",
        ">;"
    }
      .end annotation

      .annotation runtime Lretrofit2/http/GET;
    value = "card/pan"
      .end annotation
  .end method

After (v2.59.1 has the fix):

  .method public abstract pan(Lcom/monzo/card/data/api/RetrievePanRequest;)Lio/reactivex/Single;
    .param p1  # Lcom/monzo/card/data/api/RetrievePanRequest;
    .annotation runtime Lretrofit2/http/Body;
    .end annotation
    .end param
    .annotation system Ldalvik/annotation/Signature;
    value = {
      "(",
      "Lcom/monzo/card/data/api/RetrievePanRequest;",
      ")",
      "Lio/reactivex/Single<",
      "Lcom/monzo/card/data/api/PanResponse;",
      ">;"
    }
    .end annotation

    .annotation runtime Lretrofit2/http/PUT;
    value = "card/retrieve-pan"
    .end annotation
  .end method