Fonts displayed as missing with (OTF) appended even though they're installed
InDesign seems to struggle to recognise installed fonts - possibly something to do with how Skyfonts works?
A file created with Skyfonts fonts shows the font family in square braces with (OTF) appended and substitutes default font, rather than making use of the identically named font file, sitting in c:\windows\fonts\ on PC / Font Book on Mac.
Same result if the font files are dumped in a Document fonts subfolder next to the InDesign file.
Troublingly, using a different font cloud sync app (or in the rare instance where the font is available on fonts.adobe.com) doesn't remove the issue - InDesign lists those in its font family menu without the square braces and (OTF) appended.

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david wendelken commented
I created a book and individual chapter files for it. I used both Windows system fonts AND some installed fonts that I got from font websites. Worked great.
I had a computer crash. I re-installed the non-system font files.
Now InDesign does not recognize that the non-system font I used in the InDesign file is the same font as the re-installed non-system font.
It tells me the font is missing and it shows up as, for example, "Uncial [OTF]"
My guess is the font-matching mechanism does not work off the font name, but perhaps an internal Windows id for that installed font. If so, that would explain why it thinks Uncial and Uncial are different fonts.
If people who pass InDesign files to one another use non-standard fonts, it might also explain why they have problems (assuming that's also a problem).
Either way, it's a real time-consuming pain in the rear to go thru 50 files and update the paragraph and character style definitions with the same font I was using before.