That is because every time you withdraw from a credit card, there will be cash advance charges unless stated otherwise. These are the normal T&C for credit cards and despite having additional funds in your card, you cannot treat it as a bank account.

Thankfully you are with Citibank so my suggestion is that you call 03-23830000 and speak to the customer service. Tell them your case and there are chances they might be able to waive that RM50 for you.