format was broken in mpanels; some post-save functionality is missing

[aaronw22]

Describe your changes

Do the following before requesting a review

• [ ] I have written tests covering functions I have added or changed.
• [ ] I have run tests and they all pass.
• [ ] I have ensured all new functions will show up in the _pkgdown.yml file.
• [ ] I have updated DESCRIPTION with any new package dependencies.
• [ ] I have updated NEWS.md with a brief description of my changes.
• [ ] I have updated the package documentation with devtools::document().
• [ ] I have updated the package website with pkgdown::build_site_github_pages().