The following code dumps invalid YAML:

console.log(yaml.safeDump({ str: '\n    a\nb' }))

Invalid output due to indentation:

str: |-

      a
  b

Leading spaces are handled in general by an indentation indicator (below), but leading newlines seem to thwart this (above):

console.log(yaml.safeDump({ str: '    a\nb' }))

Valid output:

str: |2-
      a
  b

The results above are from version 3.11.0.