I saw that this problem has been reported (MC-133294) and fixed 5 days ago, but I found it again. (unless it wasn't put in 18w31a or 1.13 and will be put in the next snapshot)

I was making a map with 3 dark blue banners to mark on it. I named two of the banners, but not the third, the banner in the bottom left was named and placed first, then I placed the middle one, then the top right one, the two markers that are named also had framed maps next to them. I then renamed the banners trying to fix it, and placed them in the same order.