A goal of the new setup is to prevent users from writing bad YAML, but the reformatting of existing YAML is largely a side effect. YAML is horribly complicated and has numerous ways to write any given thing; Obsidian doesn’t preserve the exact formatting when it writes back to your file. From API: processFrontMatter removes string quotes, comments, types, formatting - #6 by Licat which I linked earlier:
The decision we made is for plugins to be able to interact with frontmatter easily. We do this by converting YAML into a JS object, which means it lose all of its YAML specific formatting and comments. This makes it easy for plugin developers to read and modify properties of a note, without requiring them to interact with an excessively complex YAML syntax tree, which would require them to understand how YAML is parsed internally.