Been using my AMEX for years & can't believe I have to argue with CS on M2U chat over RM0.24.
I was puzzled with the sudden inclusion of RM0.24 finance charge in my statement that doesn't appear in AMEX's transaction page on M2U. Therefore, I chat with CS right away & was given link that went TL;DR about how paying only minimum payment will incur interest. After arguing further about how I know all this, she relented & agreed to go over the transactions.

Maybank's calculation is in blue while mine is in red. Payment due date is 3-FEB.
So I have carry-over statement balance of RM546.20. Apparently, the refund by IKEA of RM149 (21-JAN) is not considered as repayment. Therefore, in Maybank's system, the total repayment I made was insufficient.

I told the CS that the IKEA transaction was made on 07-JAN & therefore, it's part of the statement balance of RM546.20. When IKEA refunded me due to their product issue, the refund should have gone into the statement balance but she said nope, it will only go into the current month's total credit.
In the end, she said she'll raise a waiver request for me but I just find it silly to go through all this for RM0.24. Oh well, here's a heads up to you guys in case you're in similar situation with refund in the near future.
The rule of thumb: always pay the statement balance on or before the due date.
Don't assume cashback, prize or credit transaction are part of payments.