Illustrator thinks font is missing, but no other application does

I have a file from a few months ago that I would like to update. When I open it in Illustrator, I get a dialog box telling me that the font it uses—Myriad Pro Bold—is missing. Yet it isn't, and no application besides Illustrator seems to think that it is missing.

• I have looked in Font Book, and Myriad Pro Bold is listed as installed and active. It's an OTF file.
• InDesign can see and use Myriad Pro Bold just fine.
• Illustrator can see and use every other weight in the Myriad Pro family. Only Bold has the little 'missing' diamond next to it.
  • When I create a new document entirely, Bold is simply missing from the list of options.
• I was given the option in the dialog to sync the font from Typekit. I tried this, and Typekit came back with the error that the font is already installed.
• I'm on Mac OS X El Capitan (10.11.6) and using the most up-to-date version of Illustrator CC 2017.
• I have tried: closing the file and re-opening it; closing Illustrator and re-opening it; restarting my computer.

What is happening and how can I fix it? Or, if not fixable, how can I work around it?

Thanks.