The bug

In multiplayer, most banner markers on maps don't show the correct symbol. This is also the case for the X mark on buried treasure maps. Out of the new markers, only the markers for white, orange, magenta, light blue, yellow, and lime banners work correctly.

This affects singleplayer worlds with opened LAN as well (the host is not affected).

To reproduce

1. Join a server or a LAN world
2. Create a map
3. For each banner color, put a banner in the area covered by the map
4. Add each of these banners to the map by using right-click
5. Notice that all except the banner colors mentioned above have an incorrect icon *
6. Run /locate Ocean_Ruin
7. Teleport to the given coordinates
8. Loot the chests and find a buried treasure map
9. Look at that buried treasure map
10. Notice that the icon is not a X, but a white banner instead

*) If you look at the map in an item frame, some banners won't be visible at all, because they're using player markers which are hidden on a map in a frame.

The reason

According to wiki.vg, the "Icon" banner field has only a byte reserved for direction and type of the icon. Because of this, only 16 map icons can be represented that way. Hence, the 17th icon (pink banner) is actually sent over to the client as if it was the first icon (player marker), the 18th as if it was the second, and so forth.

This could be fixed by changing the "Direction" part of this byte to represent the color of a banner instead (since the banner markers don't have rotation). That way, there are 11 seperate icons which fit into that byte.

Original description

My son and I were playing multiplayer LAN in 18w10a and I had the new banner markers for the maps behave strangely. I was playing on his world that he opened to LAN, so I was the joining player. I do not know if this happens for additional players beyond one or not.

Being a cartographer of sorts, I love fiddling with anything related to the maps in game. I immediately started experimenting with the banners, creating markers on a map I pulled from creative.

Here is the description of what I started experiencing:

The first six banner colors display properly: White, Orange, Magenta, Light Blue, Yellow, and Lime.

The Pink banner won't display at all.

Starting with the Gray banner and continuing with Light Gray, Cyan, Purple, Blue, Brown, Green, Red, and Black, each one displays an inaccurate marker for the banner. They include a small green, white, and blue arrow (like the ones for an item frame map and when a player is walking around), a white X, a large upside-down red arrow, the large and small white player map dots, the woodland mansion icon, and the ocean monument icon, I believe in that order.

Looking at the possible map markers on the Minecraft wiki here:
https://minecraft.gamepedia.com/Map#Map_icons
it seems for some reason that this new banner marker feature is accessing markers that are not intended to be used for the feature itself but exist in the game.

I assume this is not intended as when my son clicks the same banners I put up or the ones he put up on his own they are normal banner colors. It is only when I try to click the ones I set or the ones that he set does this happen. It seems to be an issue with me being an additional player in his world.