It first started failing under GH's Ubuntu, but it is now failing locally for me too (macOS):
$ npm run check:links--md
> docsy@0.11.1-dev-unreleased check:links--md
> ls *.md | xargs npx markdown-link-check --config .markdown-link-check.json
FILE: CHANGELOG.md
FILE: CONTRIBUTING.md
FILE: README.md
$ echo $?
1
There's an issue with markdown-link-check.
Might it be time to tackle #1781 ? :)