Strange behaviour with unquoted dot

[KevinMitchell]

In a YAML source A I define

openapi: 3.0.0
info: ...
components:
  schemas:
    Foo:
      type: object
      properties:
        ...
      x-foo:
        bar: .

In the editor there are no markers. In a separate YAML source B I reference a schema in source A. It complains with an invalid reference error. It does this even if I'm accessing a schema other than Foo in A. If I change the first source to quote the dot

      x-foo:
        bar: '.'

the marker goes away and it's happy with all references to schemas in A. Is the first version valid YAML, in which case why is a marker being generated? And if it's not valid YAML then why don't I get a marker shown in A?