Setting strategy.matrix to variable value returns a "invalid workflow file" error

Describe the bug It seems that we can no longer set matrix.strategy to an output of a previous job. Trying to do so will cause the job to immediately fail with a “Invalid Workflow file” error.

To Reproduce

1. Use the example from the Context and Expression syntax guide from the GitHub Actions docs (https://docs.github.com/en/free-pro-team@latest/actions/reference/context-and-expression-syntax-for-github-actions#example-6)
2. Push up to any repo
3. The workflow file error will come up immediately

Expected behavior There should not be a workflow file error unless something has changed about matrix variables/syntax

Runner Version and Platform

Version of your runner? ubuntu-latest

What’s not working?

An example can be found here The error text is: “Invalid workflow file You have an error in your yaml syntax on line 15”

Job Log Output

No job log output as the jobs never run

Runner and Worker’s Diagnostic Logs

No diagnostic logs available

For reference here is the example from the docs pasted fully:

name: build
on: push
jobs:
  job1:
    runs-on: ubuntu-latest
    outputs:
      matrix: ${{ steps.set-matrix.outputs.matrix }}
    steps:
    - id: set-matrix
      run: echo "::set-output name=matrix::{\"include\":[{\"project\":\"foo\",\"config\":\"Debug\"},{\"project\":\"bar\",\"config\":\"Release\"}]}"
  job2:
    needs: job1
    runs-on: ubuntu-latest
    strategy:
      matrix: ${{fromJson(needs.job1.outputs.matrix)}}
    steps:
    - run: build

[cdb]

We have rolled out 2 bug fixes now, to cover both the cases in this issue (matrix: ${{ expression }}, and strategy: ${{ expression }}, and have added additional unit test coverage. Thanks for reporting this and let us know if you continue to see the issue!

[sindrig]

According to @cschleiden this should have been resolved. See discussion here: https://github.community/t/dynamic-matrix-yaml-syntax-error/136705/7

We would like some kind of an explanation and/or what will be done to prevent something like this again.