Even if Obsidian doesn’t support multi-level YAML, currently if you have a list of lists as [ ] it expands it into - - format so it imposes some kind of structure. That still doesn’t allow it to be used.
Either it should leave this kind of YAML alone or handle it fully. The software now screws with YAML that it reports it doesn’t recognize, and then it still doesn’t recognize it after doing so. What’s the point of that exercise?
The worst part is that any edits in Bases are treated as an update to the YAML in the same way, causing it to screw with multilevel YAML thus expanding lists of lists. This makes it basically impossible to keep organized multilevel YAML readable for use in dataviewjs, because all conveniences related tangentially to YAML break it.
Either do nothing or implement it fully, don’t enforce unwanted structure into YAML that you’re not handling anyways
EDIT:
(My bug report: Non-Source mode property edits detrimentally convert flow- to block-style (multi-level YAML, list of lists, sequence of sequences, ..))
(Apparently a related Dev/API post: API: processFrontMatter removes string quotes, comments, types, formatting)