Flag management in yaml not working

https://docs.codecov.com/docs/flags#step-2-flag-management-in-yaml
I tried this configuration:

flag_management:
  default_rules: # the rules that will be followed for any flag added, generally
    carryforward: true
    statuses:
      - type: project
        target: 90% # overall project/ repo coverage
      - type: patch
        target: 90%
  individual_flags: # exceptions to the default rules above, stated flag by flag
    - name: backend
      paths:
        - backend/
      carryforward: true
    - name: webapp
      paths:
        - webapp/
      carryforward: true

and also this configuration:

flag_management:
  default_rules: # the rules that will be followed for any flag added, generally
    carryforward: true
    statuses:
      - type: project
        target: 90% # overall project/ repo coverage
      - type: patch
        target: 90%

flags:
  backend:
    paths:
      - backend/
  webapp:
    paths:
      - webapp/

and both of them is not working and UI don't show any flags

Start with the linked flag-management documentation and compare its YAML examples with the two configurations reported here. Then trace how the GitHub Action passes flag configuration to Codecov and reproduce the missing flags; done means a valid configuration makes the backend and webapp flags appear in the Codecov UI.