When some nested property is added using the "Content Assist" tool of the YAML editor (STS 4.13.0), that is immediately reported by the validator as an error, such as "Nested mappings are not allowed in compact mappings" or "Implicit keys need to be on a single line".

As I found out, that error is caused by the EOL characters generated by the YAML editor, which follow the Linux convention (CR). However, I am running on Windows, and also installed the Windows version of STS (spring-tool-suite-4-4.13.0.RELEASE-e4.22.0-win32.win32.x86_64.self-extracting.jar).