My iPhone is showing my daughter's name on texts from both her and her dad’s numbers

TL;DR: My daughter's contact name is associated with two phone numbers in my phone and I can't make it stop!

My daughter and her dad recently upgraded their iPhones and switched providers. During the transfer process, she sent me a text from her new phone, but it came through from her dad's number. I pointed this out and she was confused and then they asked the T-Mobile guy to fix it, which he did. Ever since then, her name shows up on texts from her number AND from her dad's number. I've deleted both contacts from my phone and restarted. I still get texts from both of them with her name attached. Additionally, when I start a new text message (say if I want to text a photo) and type her name into the To: field, her dad's number comes up as one of the options. I have been accidentally sending him messages all week thinking they are going to her.

I've had them both check Settings > Phone > My Number to make sure it's the correct number, and it is. They've also both checked Settings > Messages > Send & Receive to make sure their numbers and Apple IDs are correct.

What else can I do?! Is this a T-Mobile issue or ...?? BTW this is only happening to me, doesn't seem to be an issue for others who they both communicate with.

[Re-Titled by Moderator]

Original Title: wrong contact name associated to phone number