If two players have a filled map item of the same map ID, it will not show either player's markers on the map if the map has a custom color or display.

I also noticed if there is a map with a custom display tag in an item frame, it causes the player to blink in and out of existence on the map.

Note the player is red on the map marker in the image below due to a resocepack, when turning this off, the bug still happens I just forgot to when taking the screenshot.

I should probably add that I'm using Minecraft realms when this happens, I have not tested it on a vanilla server.