This happened in my creative world. When I was not on the same level as the banner and the top part of it was not on screen, the bottom part wasn't visible too. That only happened when I was more than 1.5 blocks away from it. If I stood about two blocks away from it, the banner just despawns at certain angles.

Screenshot 1: On the same level as the banner, top and bottom visible.

Screenshot 2: On the same level as the banner, top not on screen, bottom visible.

Screenshot 3: One block lower than the banner, top and bottom visible.

Screenshot 4: One block lower than the banner, top not on screen, bottom invisible.

Screenshot 5 & 6: Different angles effect the visibility of th bottom part when I'm close to it, even though the top is not on screen in both.

Edit: I haven't tested it out yet, but it seems like it's because of subchunks and maybe on other layers, it'll work differently. In these pictures,  I'm somewhere between level 60 and 64.