Default config does not report diffs, even though --diff appears to be executed

[elliot-100]

Docs: https://pycqa.github.io/isort/docs/configuration/github_action.html and https://github.com/isort/isort-action/blob/master/README.md say:

isort configuration options to pass to the isort CLI. Defaults to --check-only --diff.

Expected behaviour:

Output including diffs, similar to what I get running isort --check-only --diff . at CLI, e.g.

isort --check-only --diff .
ERROR: C:\Users\[REDACTED]\code\elliot-100_Spond\tests\test_spond.py Imports are incorrectly sorted and/or formatted.
--- C:\Users\[REDACTED]\code\elliot-100_Spond\tests\test_spond.py:before        2024-05-22 10:26:37.165779
+++ C:\Users\[REDACTED]\code\elliot-100_Spond\tests\test_spond.py:after 2024-05-22 10:59:23.726940
@@ -1,7 +1,6 @@
 """Test suite for Spond class."""
 
 import pytest
-
 
 from spond.spond import Spond
 
Skipped 2 files

Actual behaviour:

Output does not include diffs. Extract from GitHub actions log. Note --diff does appear:

Running isort --check-only --diff .
Error: ERROR: /home/runner/work/Spond/Spond/tests/test_spond.py Imports are incorrectly sorted and/or formatted.
Error: Process completed with exit code 1.

i.e. similar to output I get on running without the --diff flag at CLI:

isort  --check-only .
ERROR: C:\Users\[REDACTED]\code\elliot-100_Spond\tests\test_spond.py Imports are incorrectly sorted and/or formatted.
Skipped 2 files

Please let me know if any extra information is required.

[elliot-100]

After discussion at #72 :

Workaround:

Don't use official isort GitHub action, use command line interface, e.g:

steps:
  - name: Lint imports with isort
    # No modifications; exit with error if the code is not properly formatted; show diffs
    run: isort . --check-only --diff