NBU get update via the robot of the tape library. So, you can only run inventory after the physical inventory of the library completed (when you see 'unlock' on the library LCD panel) and not once you close the CAP.
When you closed the CAP (with TapeB in CAP slot), i believed the robot have somehow released the memory (not too sure if this is by design), and if you run NBU inventory immediately, NBU will update it media database from the robot and sent TapeA or all tapes to standalone drive
Once the physical inventory completed, the robot updated the new information in the memory and u get the correct info when you run NBU inventory now.
I'm using 3853, the physical inventory took about 3mins to 4mins but when i run inventory in NBU, it get the updates in about 10secs.
I have ever did an implementation on IBM 3854 library using Legato NetWorker. I get the updates in matter of second when i run inventory in NetWorker.
Not too sure for 3585. I suggest you check with IBM or VERITAS.
Remember, you can only run NBU inventory after the physical inventory of the tape library completed