It is super frustrating as it means any replies to text messages from her go via the MMS gateway (not SMS) and are sometimes delayed by a long time.
In my case, I'm a T-Mobile subscriber on an Android Nexus 5 and my number was ported from Verzion Wireless. The contact I'm having trouble with is my wife who is still on Verizon.
If I delete the thread from her and then delete the contact for her and I send her a text message just to her number, it shows up as a thread associated with her number until I go back to the list of threads and then come back to this thread. Then it shows as <number>@vtext.com. (This happens even when not having received a subsequent text from her.)