How to fully remove number associated with a contact?

My wife has two iPhones. The text strings keep getting combined so I've deleted her work phone number from her contact profile. Unfortunately, Apple doesn't want to let her go. Even if I text her personal number, it still shows up as her deleted work number on my phone. I have her profile pinned in my messages but it gets unpinned frequently. Her profile picture and name disappears and only her deleted work number is displayed even though I'm texting her personal number. How can I completely disassociate her old number from her profile?