some items placed on the end obsidian platform vanish when another player comes thruogh

I placed an ender chest, a shulker box and 4 item frames with maps on the obsidian platform created when entering the end.  Another player came through and the ender chest and shulker box (and its contents) vanished.  I had a second ender chest and placed it in the same location and had a player come through again and it vanished again.  If it matters the box was in the SW corner of the obsidian platform and the shulker box was next to it to the north.  The item frames were not at the edge of the platform and didn't vanish.