If you have a YAML file with the following content:

username: |
    Bob

The parsed result will be:

[
  "username" => "Bob\n"
]

Which is expected. If you use instead:

username: |-
    Bob

The result will be:

[
  "username" => "Bob"
]

Up to that point, no problem. Now if I have:

User:
    foo:
        username: |
            Bob
    bar:
        username: |
            Bob
        group: ~

I would expect:

[
  "User" => [
    "foo" => [
      "username" => "Bob\n"
    ],
    "bar" => [
      "username" => "Bob\n",
      "group" => null,
    ],
  ]
]

But instead, of having "Bob\n" for "foo", I get "Bob" instead.

I'm not sure if it's the expected behaviour, but looks odd to me. Bug reproduced here. Tested on the the version v3.0.6.