I would suspect a session timeout on the Omron side was still holding the items from before the restart, and therefore rejected the post-restart subscription requests. Which were then poked/prodded when you added the one item a bit later.
Vendors put limits in these on-PLC OPC servers to protect the device against overload (OPC servers are pretty heavy-weight tasks for true real time environments), but those limits have side effects. If you can limit using the OPC connection, you should.
FWIW, I'd be delighted to have more NX testing of my EtherNet/IP client driver, which supports the NJ/NX family:
/shameless plug