Execute the following code in the debug console on a dedicated server:
_uniform = uniform player;
removeUniform player;
player addUniform _uniform;
Items on the local players vest will be gone according to the items command.
If you look at the items of that player on a remote machine or on the dedicated server you will see that the items that were in the uniform are still there.
Executing addItemToUniform will show item added locally but it will fail to appear on any other client or the server.
You can exchange any of these commands for their vest of backpack equivalent and experience the same issue.