Reference to multiline string rendered incorrectly in output

**Describe the bug**

Consider the following input config:

```YAML
SOME_VALUE: |
  this is
  a multiline
  string
SOME_OTHER_VALUE: ${SOME_VALUE}
```

This will render as invalid YAML:
```YAML
SOME_VALUE: |
  this is
  a multiline
  string
SOME_VALUE: this is
a multiline
string
```