AFAIK, that's normal. Most auto trannys do that (even on my toyota camry). My RPM drops as well when I let off on the gas. Everything is electronically controlled now. The ECM decides when to lock and unlock.

Just thinking out loud, maybe the reason for TC unlocking when you release the gas pedal is that in the pre-ABS days, you might slam and lock the wheels in an emergency situation. And if your TC is lazy or sluggish, it may not unlock in time and you could stall the engine making you lose power steering, and make the truck harder to control. So maybe releasing early (between letting go of gas and slamming the brakes), this can be avoided. Nowadays with ABS, though possible, wheels are harder to lockup. Nissan may have decided to leave this design in there just for safety reasons since all other auto manufacturers left it in as well.