Weird volumes with numeric names

Every time I run docker-compose up, it creates an additional volume that won’t be in use the next time when running the command. It seems like it is a temporary volume or something that doesn’t get deleted when running docker-compose down.
Any idea why and how to avoid it?

Did you name your volumes?

Yes, I call it vidly and Docker gets that name from the last line of the docker-compose.yml file.

version: "3.8"

services:
  frontend:
    depends_on: 
      - backend
    build: ./frontend
    ports:
      - 3000:3000

  backend: 
    depends_on: 
      - db
    build: ./backend
    ports: 
      - 3001:3001
    environment: 
      DB_URL: mongodb://db/vidly
    command: ["sh", "docker-entrypoint.sh"]

  db:
    image: mongo:4.0-xenial
    ports:
      - 27017:27017
    volumes:
      - vidly:/data/db

volumes:
  vidly: