This is recent (popped up in the last week), and I don't think it came up with any updates; most recent thing I can think of is watchOS 4.2. I'm on the iPhone 7, with iOS 11.2.
Only in my Messages app, some contact names are still visible, but most contact names have been replaced with their phone numbers. The contact "picture" on the side still has their initials though.
Any ideas on how to fix this?