Is there any issue happening on codecov.io server side?
Sometimes it reports 400 error while running in GH workflow:
HTTP 400
ERROR: The coverage upload was rejected because the workflow run is either not in-progress or finished a while ago. Please make sure the workflow run is in-progress and is not stale for uploads to process correctly.
More details here: error: upload was rejected because the workflow run is either not in-progress... · Issue #68 · codecov/codecov-action · GitHub