If refeshing the image fails for whatever reason (couldn't connect, not found, etc), the image isn't updated. The system will then rely on the the lifetime (TTL) to handle when it would be removed. If that's set to permanent, then the image wouldn't be removed.
In the database, there is a failed_date
and fail_count
value stored for image proxy items. This also doesn't track the why -- it could be a temporary network issue, for example. But there isn't anything in the UI that exposes this information.