Players not on cloned maps in shulker boxes

When a player has a map in their inventory, they are visible to other players with a clone of the same map. This does not appear to work if the player has the map stored inside of a shulker box in their inventory. Strictly speaking, items in a shulker box are still in the players inventory, so it seems like it should still display them on the map. Is this an intended feature, or just something that has never been thought of since shulker boxes were added?

Note: Tested on a 1.15.2 vanilla server, and LAN world.