Spot the Bug - Typescript

CREATE TABLE "template" (
  "id" TEXT NOT NULL,
  "startTime" TIMESTAMP(3) NOT NULL,
  "price" DOUBLE PRECISION NOT NULL,
  "listId" TEXT NOT NULL,

  CONSTRAINT "template_pkey" PRIMARY KEY ("id")
)

CREATE TABLE "concrete" (
    "id" TEXT NOT NULL,
    "listId" CITEXT NOT NULL,
    "startTime" TIMESTAMP(3) NOT NULL,
    "price" DOUBLE PRECISION NOT NULL,
    "customerId" CITEXT NOT NULL,

    CONSTRAINT "concrete_pkey" PRIMARY KEY ("id")
);

CREATE UNIQUE INDEX "concrete_id_startTime_listId_key" ON "concrete"("id", "startTime", "listId");

interface Template {
  id: string;
  startTime: Date;
  price: number;
  listId: string;
}

interface Concrete {
  id: string;
  startTime: Date;
  price: number;
  listId: string;
  customerId: string;
}

async function getAllTemplates(): Promise<Template[]> {
  return query('SELECT * FROM template');
}

async function getAllCustomerIds(): Promise<string[]> {
  return Promise.resolve(['1', '2']); // no query here for simplicity
}

async function createConcreteValues(values: Omit<Concrete, 'id'>[]): Promise<void> {
  // insert values into DB
}

async function applyTemplates() {
  const allTemplates = await getAllTemplates();
  const allCustomerIds = await getAllCustomerIds();

  const concreteValues: Omit<Concrete, 'id'>[] = allTemplates.flatMap((template) => {
    return allCustomerIds.map((customerId) => ({
      ...template,
      customerId,
    });
  };

  // this call will throw a unique constraint violation exception; why?
  await createConcreteValues(concreteValues);
}