The bug

Marked banners and placed item frames show up in different orders after the game is reloaded. For example, here are how multiple placed maps in the same block facing opposite directions behave https://gyazo.com/c7d4bc25097f06b9a155e8f6f9ec4e58 

When the game is exited and re-opened however, much of the movement of the markers stops entirely https://gyazo.com/c7d4bc25097f06b9a155e8f6f9ec4e58

Here are some of the issues that can be seen with marked banners on maps. https://imgur.com/a/H04zHNO

These issues are pretty annoying when trying to create specific effects on a map since the text from banners will become covered up by other banners and the order of the banners is not consistent.

To reproduce

1. Create a map
2. Place multiple banners near to each other
3. Add all of them to the map, so that their icons overlap each other
4. Reload the world
   → Notice that the order in which the icons are rendered changed