Mmh…not sure what you want to do.
Have a multiline string in a frontmatter field?
Goldmark is processing the markdown, not YAML.
The frontmatter is parsed by hugo and so a data file would.
Multiline definitely works with hugo and goldmark. See this topic from 2022. Rendering of string using markdownify not working
Guess It would be good to provide an example code that shows your problem.