Yeah, great question. It has to be valid YAML or else the frontmatter won’t parse properly. But apart from that Obsidian is very forgiving. The following all work for me:
tag: one, two, three
tags: one, two, three
tags: ["#one", "#two", "#three"]
tags:
- one
- two
- three
A rule of thumb: Don’t to use the # symbol in the metadata unless you enclose the tag in quotes.