Playground does not catch certain Shellcheck issues

[stackptr]

With .github/workflows/test.yml:

name: Test workflow

on:
  push:

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - run: |
          now="$(date +%s)"
          read -r -d '' req_json <<MALFORMED_EOF
          {
            "foo": "bar"
          }
                MALFORMED_EOF
          echo "$req_json"

Copying and pasting into the playground yields no issues in Safari or Chrome.

Running actionlint (via Nix) in CLI catches an issue with malformed here document:

shell ❯ actionlint .github/workflows/test.yml
.github/workflows/test.yml:10:9: shellcheck reported issue in this script: SC1009:info:2:1: The mentioned syntax error was in this simple command [shellcheck]
   |
10 |       - run: |
   |         ^~~~
.github/workflows/test.yml:10:9: shellcheck reported issue in this script: SC1073:error:2:24: Couldn't parse this here document. Fix to allow more checks [shellcheck]
   |
10 |       - run: |
   |         ^~~~
.github/workflows/test.yml:10:9: shellcheck reported issue in this script: SC1039:error:6:1: Remove indentation before end token (or use <<- and indent with tabs) [shellcheck]
   |
10 |       - run: |
   |         ^~~~
.github/workflows/test.yml:10:9: shellcheck reported issue in this script: SC1072:error:9:1: Here document was not correctly terminated. Fix any mentioned problems and try again [shellcheck]
   |
10 |       - run: |
   |         ^~~~

As far as I can tell, there is no configuration that actionlint is using locally that would explain these different outputs.