http://feedback.arma3.com/view.php?id=19829 is still a bug.
If you remove and re-add the same uniform/vest/backpack in the same frame (non-scheduled SQF execution) the gear system corrupts and doesn't register the container with clients or the server, and any subsequent calls to addItemToUniform/Vest/Backpack are corrupted and do not register on anyone but the local machine in which it was called.
Introducing a delay fixes the issue, but this is a hacky workaround.
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.
Only happens when removing and re-adding the same uniform/vest/backpack and when there is no delay.
This is a serious bug that can cause unexpected behavior down the line as it branches the inventories from player to player and from player to server causing inconsistencies.