What it means when one contact is showing as number in Messages?

Am I blocked? Contact name not populating

One particular contact keeps swapping between their phone number and their name in the list of messages.

What happens is l'll scroll down to my conversation with this particular person, and their name is being displayed, but then when I go into our conversation and click on the name up top (where you would click if you wanted to see photos from your messages, etc.), I see their phone number displayed. I included pictures that might make this make a bit more sense if it's confusing what l'm trying to say. And also this will randomly happen every now and again, their contact doesn't display their number like this.

Has this person blocked me? Why is this happening with only 1 person?

[Edited by Moderator]