We are working on finding a fix for this issue.
Adobe InDesign team
An error occurred while saving the commentDaniel commented
I saw that InDesign 15.1 came out today and updated to it, but I still had the problem with InDesign not finding a font family I use all the time. Out of frustration, I contacted Adobe support and was told that since the font was not an Adobe font, I should contact the font maker. I knew there wasn't a problem with the font because the font worked fine in the latest PhotoShop 21.2.0 which I installed today, and it worked fine in all my other applications.
I checked my InDesign setup and saw that I had the font Iowan Old Style.ttc in both the Adobe InDesign 2020->Fonts folder and the Library->Fonts folder. I removed the copy from the Adobe InDesign 2020->Fonts folder, restarted Adobe InDesign 15.1, and the problem with InDesign not finding the font went away. I am relieved because I use that font in hundreds of documents, and if InDesign could no longer find the font, I was looking at finding an alternative to using InDesign.
InDesign versions up through 15.0.1 had no problem dealing with a font being in more than one folder, but starting with version 15.0.2, InDesign, including version 15.1, couldn't handle the setup.
So, if you are having the similar problem, you might check to see if you have the font(s) InDesign can't find in more than one Font folder on your system.
I am using InDesign 15.1 on a MacBook Pro running MacOS 10.15.5.