Avatars broken for one user

So here's an interesting issue: A user complained that the default avatar was appearing for them, even though they had uploaded a new one multiple times. I figured this was operator error, or a simple matter of clearing their cache. However ...

I re-uploaded their avatar, and the new one displays fine on the avatar upload page. But the default still appears next to all their comments. AND, the default appears next to their username on the left when searching for the user in the admin, but the new one displays to the left under avatar! See attached screen shot.

And here's the interesting part: Avatars are still working properly for other users. And I uploaded new avatars for two other test user accounts and they work as expected. For the trouble user account I tried various different files and formats, all with the same results – new avatar shows fine on the upload page, but nowhere else.

Any thoughts on how I might troubleshoot this one are greatly appreciated. I confirmed proper permissions settings on related directories, but don't know what else to try. Where might I look in the database to try and fix this from the backend?