How much did you send? I have done few separate transactions deposit RM500 > XRP > XRP to external exchange wallet no issues.

could be large amount trigger auto lock or maybe random lock as part of security to stop pipits from washing money.

Over RM10k probably trigger it. I assume that you transfer MYR from you bank account to luno which is why they manually unlocked you so fast. If the other way around, no transfer MYR in from bank before, but suddenly got Crypto coming in to cash out then they will ask for more proof.

I don't think that it is Luno purposely locking you. Likely is BNM or SC directive to scrutinise a certain amount of transfers and so you were automatically flagged by system.