Hello this bug report was originally posted in dhall-lang/dhall-haskell#1939 (comment) byt the incorrect configuration (with dhall-to-yaml --version v1.7.1) seem to be because of a change in aeson-yaml (which removed the comment warning about the : string sequence. See the bug in action:

> dhall-to-yaml --version
1.7.1

> dhall version
1.34.0

> dhall-to-yaml <<< '{ headers = ["Location: https://example.com"] }'
headers:
  - Location: https://example.com

> dhall <<< '{ headers = ["Location: https://example.com"] }'
{ headers = [ "Location: https://example.com" ] }

The YAML generated value is a Hash containing a List containing… another Hash!
Whereas the normalised form output from dhall is (correctly 😌) a Hash containing a List containing a String.