Oh, fer' cryin' out loud! How many fonts is Apple going to goof up?
It's not anything you're doing. Starting with El Capitan, these five fonts have (almost) never worked as they should in any Apple supplied app.
Athelas.ttc
Charter.ttc
Marion.ttc
Seravek.ttc
SuperClarendon.ttc
In High Sierra, the only way to make them work is to copy those fonts from a Yosemite install and replace the ones installed by the OS.
Now we get to add Iowan Old Style to the list. And it literally is only Apple's apps that won't see them. Not TextEdit, Pages, you name it. Here's TextEdit's list with the font active:

Nowhere to be seen. But Word 2016 (below), and any other third party app has no trouble seeing and using all of these "hidden" fonts.

What is Apple doing to cause their own apps to refuse to see and use certain fonts the OS installs? A poorly written function related to System Integrity Protection that is seeing the newer versions as infected? Who knows.
Like the five other fonts mentioned, I copied Iowan Old Style from Yosemite, and now it works everywhere, as it already should.
I'm going to have to check every single OS supplied font again and add the ones that don't work to my font article.