I'm also experiencing the problem, and can add more detail. First, I have a subscription to both the NY Time Newsletter and their Morning Addition under an @yahoo address - both are NOT formatted correctly. I also have a Morning subscription under an @gmail address - that one IS formatted correctly. Both the yahoo and gmail accounts are setup as IMAP on all my devices.
By "formatted correctly" I mean that at first, portions of the body of the email appeared, but most of the content was missing. About a week ago the behavior changed on now only the header info (to:, from:, etc) show - there is no content displayed.
At first, the badly formatted emails displayed a link to "open in browser" (works perfectly). About a week ago that option also disappeared.
Before you place the blame on Yahoo - note that Apple Mail on iPad and iPhone works as expected.
Thanks for your attention!