did it used to?

you missed all the fun during 1 Sep revision 

I don't think it has ever given cashback for petrol...

I traced back the thread, the person was recommending you to 'wave' the Quantum Visa card at petrol station counter with minimum RM30.00.  That's how he suggested to you on how to get cashback for petrol.

yes installment but per transaction basis.

min RM1.2k per transaction, call CS to convert to 0% 12mth Flexipay (before 1 Dec 2019)

yes

flexipay is post-purchase conversion to installment.

Yes, call cs to change it.

read the T&C on how the sms registration vs spending is being tracked:

Can settle the outstanding balance on 9th after I got salary during month end.

Nope. No issue and in fact I just set it up again on my new phone yesterday, able to login with no issue.
Just tried logging in after seeing your post and all goes well.

Maybe uninstall and reinstall again?

Apart from the salary thing, somehow I will always have some spending on or before 20th, so I'll usually wait till 21st to swipe the card. let's say I spend on 21st Dec, then the payment will be only settled on 9th Feb 

So I even purposefully changed my statement date to 20th for new cards.

errrr why did you assume "most people" are having statement date middle of the month?

everyone has different needs. some dont care.

usually, the primary reason is to coincide the due date slightly after salary day (again, not everyone's the same, but usually 4th week or even 1st week of the following month!)

if you are a good paymaster, you gain the +20days grace period for payment.

e.g.

gaji day = 29th
cc statement date = 9th
cc payment due date = statement date + 20days = 29th

ngamngam you pay the cc on your salary day.
again, some also dont care and some immediately pay after statement date (those dont feel like having outstanding).