iOS 9 iMessage notifications show sender's number, not name

After some annoying wifi problems with iOS 8.3, I tried out iOS 9 public beta 2 (now public beta 3) on my iPhone 5 (Edit: in the UK). When I receive a text message (iMessage/SMS), the lock screen/home screen notifications usually come up with the sender's phone number, not their name. This happens for most senders, even though their names and numbers are in my contacts. When I open the Messages app, all messages are correctly categorised according to the contact's name, so my problem is only with the notifications. For some reason, notifications work correctly for one contact, but no others.

Questions:

• Is this a known bug?

• How can I work around it?

On cause of numbers showing up instead of names is if there is a cached phone-number-only contact that needs to be removed.