db.t3.micro Reserved Instance

Hello,

Please refer to link[1] which mentions "Size-flexible reserved DB instances can only scale in their instance class type. For example, a reserved DB instance for a db.r5.large can apply to a db.r5.xlarge, but not to a db.r6g.large, because db.r5 and db.r6g are different instance class types.". So in this case db.t3 and db.t2 are different instance classes which most likely is the reason you are getting charged for db.t2.micro. I will recommend to reach out to AWS billing team regarding further inquiries.

References: [1] https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/USER_WorkingWithReservedDBInstances.html#USER_WorkingWithReservedDBInstances.SizeFlexible