My understanding (backed up by my experience) is that while in theory NAB has finalised purchase of the old Citi portfolio in reality integration of the two is far from complete. In particular, my experience (and what commentary I’ve found) would suggest both systems and processes using those systems are yet to be fully integrated. This appears to tally with your experience!Adding some data to this thought from a while back...
I just successfully applied for Virgin Money and NAB cards. The questions asked during the application processes were different, with the NAB form requiring more details about income and spending patterns. I applied simultaneously but approval took one day for VM and five days for NAB.