That answer and its question were posted by the same person, using two different accounts. When this was discovered, moderators deleted the sockpuppet account and the owner of the other account requested that one be deleted as well.
So in a sense, the system was doing the right thing here - not pinning a self-accepted answer - but, it was doing so for the wrong reason: with both authors gone, the relevant comparison boiled down to,
if ( null == null )!
Naturally, there are thousands of other cases where both authors are gone but without sockpuppets being involved. In those cases, it's clearly undesired behavior to unpin... So starting today, those will behave normally unless the system knows the authors were the same.