I think its always been that way?
I found that I had to turn off then turn on iMessage and FaceTime on my iPad before my mobile number would appear on it. If it's not showing up for you give that a try.
Do you old messages get synced by any chance? It would be great to keep the conversations on both devices, and I'm wondering if this would solve the issue of transferring txt messages if you do a clean install or restore of one of your devices.