IWBUMS 39.64.1 on Windows.
In multiplayer, items don't always show up in containers for all players.
I'm playing a two player PZ game in which I am the host, and hosting the game though the in-game menu in the client (as opposed to dedicated server). Sometimes, I will not be able to see items that the other player can see in a container. If the other player takes the item out of the container and puts it back in, I can now see the item. There have been no cases (so far) of items that I can see that they can't.
This problem has happened with both items that were generated in the world (i.e. canned goods in a house we were looting) and items that we placed (water in a cooking pot in the stove in our safehouse).
We've been playing quite a bit since vehicles were introduced in IWBUMS, but this has only been a problem since 39.64.