I think that custom formatting of specific forms leads you down some pretty tricky paths.
I’m not as concerned about the formatter slowly evolving over time. You definitely want to pick an approach that you can stick with and not be reformatting everyone’s code every three months. While the spec is being developed and adopted there will be bugs or new situations that haven’t been considered and you would like to be able to address them. I could see an argument for setting the threshold for making changes quite high though.