I'm on my phone and have gone thru a ton of pages on this thread, so please forgive me if this dead horse has already been beaten....
Since the update, all my Twitter accounts were unlinked from their Outlook counterparts and vanished from my 1020. I can still see them linked in the Windows 8.1 People hub. The accounts that weren't linked aren't missing. However, if I link them via Windows 8, they leave my phone. If I link them on the phone, they stay.
And here's the kicker. I unlimked then unfollowed several twitter profiles. Waited a day, then re-followed them. The accounts that had vanished earlier are still missing!! Yet again, they are on my Windows 8.1 People hub.
(and because it should be the first thing most of you think of, yes, I double checked I wasn't filtering my contacts or hiding those without phone numbers.)