GitHub ci-test throwing "Your lockfile needs to be updated" even after I committed an updated Yarn lockfile

I was trying to merge my pull request to a repository. I got this error:

error Your lockfile needs to be updated, but yarn was run with --frozen-lockfile.

I figured this was due to my changes including new packages, so the lockfile has to be updated. So I pushed my modified lockfile and updated the pull request, then fixed the merge conflict on the lockfile on GitHub and committed the changes. However, the new commit still throws the exact same error. What am I doing wrong?

My ci-test.yaml file:

name: ci-test

on:
  pull_request:
  push:
    branches:
      - main
      - dev

jobs:
  test:
    name: Test
    runs-on: ubuntu-latest

    services:
      postgres:
        image: postgres:14

    steps:
      - name: Check out repository code
        uses: actions/checkout@v3
      - name: Use Node.js
        uses: actions/setup-node@v2-beta
        with:
          node-version: '16'
      - name: Install dependencies
        run: cd backend; yarn install --frozen-lockfile;
      - name: Run tests
        run: cd backend; yarn test:e2e --force-exit;

I tried cloning a fresh copy of my forked repo, checked out the branch I was working on, then did cd backend then yarn install —-frozen-lockfile as specified by the ci-test.yaml file… and it worked perfectly fine. So why won’t it work on Github Actions???

It turns out that the GitHub Action VM was using Yarn 1.22.19 while I was developing locally with Yarn 3x, which made the lockfile different so it had to be modified when installing dependencies on Github Actions. I ended up changing to Yarn 1x locally instead.