I emailed to YES on some queries, they replied to me after 3 days

Anyway I asked them to give more details on the MUR, this is their reply

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Thank you for contacting YesCare.

With reference to your enquiry, The minimum useage is Rm30 a month, if you top up Rm20 and do not use it at all, at the end of the month there will not be any balance left. However you are required to make the payment of Rm30 monthly as that is the Minimum Useage Requirements. Failing to do so you it would fall into the negative credit account.

Please do not hesitate to contact us again if you have any further inquiries.

Thank you for choosing Yes!

Always at your service,
Customer Advocate
YesCare

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So yeah, credits DO expire if you do not use RM30 worth of credits