In CI, YAML anchors don't work with include

Zendesk ticket (internal link): https://gitlab.zendesk.com/agent/tickets/93260

When using a YAML anchor in .gitlab-ci.yml and linking to a job defined in an external file, we get the following error:

Found errors in your .gitlab-ci.yml:
Unknown alias: shared-install

Example error: https://gitlab.com/harish_test_group/ci_include/pipelines/19596761/builds

I've confirmed that the contents of the external file ARE read (see previous job), but the anchor isn't. Is this currently expected? Or is it a bug?

/cc @bikebilly @nolith