About is a single template page, originally I had it as /content/about.md but have also tried /content/about/index.md neither seems to make a difference.
Your code is actually the same as I’ve used on a project. The only other differences I can spot apart from the slash are -
I don’t have any dashes in the template statements {{- …}
I have {{ $currentPage := . }} declared right above {{ range .Site.Menus.main }}, but that’s for something else in the menu
I use /content/blah/_index.md for each section page as they’re branch and not leaf bundles but it sounds like using index.md works for you because it’s meant to be static https://gohugo.io/content-management/page-bundles/ ?
I managed to get the font-bold class applied the About page by taking the menus out of the config.toml file, and moving them into the frontmatter of each section.
menu = "main"
weight = 2
It’s not ideal, but it works. Would still love to know why it wasn’t working before. The repo can be found here if anyone feels like looking for themselves.