This isn't a question. I'm sharing an issue i verified with Apple support.
If you have M4a Files in your local library on a Mac (Catalina) and have those files present in a Cloud library playlist, i have found that even though those files show and download ok on iOS, they don't play.
I have tested with multiple files.
Solution is to convert to MP3, add to library on Mac, update cloud library and then iOS works as expected.
Ironic that the problem is with Apple's own AAC format 🙂