Donor with only email address shows no display name
This sounds like a bug in recent CiviCRM, especially given that you've confirmed the behavior change on demo site.

Why this happened isn't so important I expect as what to do to get it fixed, and whether this change in behavior is intended - I'd guess not.

To get it fixed, the next step would be to open an issue in the bug tracker at http://issues.CiviCRM.org - your details here are perfect for that.

You or your organization could contribute further towards the fix, but a thorough bug report like you've provided is a great contribution already.