Single EC2 Instance Falling under both Savings Plan & RI
We have two 3 year no upfront ComputeSavingsPlan & 3 year no upfront Reserved Instance plan (Any region). For Oct billing, I see some EC2 instances are billed under both RI & SP. From the documentation it says, first RI will be applied and then Savings Plan. But what I am observing, for first few hours during the day, the RI getting applied and then Savings Plan and then Reserved Instance which is quite unusual. 
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I think the key point here is, these assignments are not permanent but can change over the time. It would be normal to see an instance utilizing first an RI and then later a savings plan, and maybe RI again. RIs are applied first but if they don't cover all instances, rest will get a savings plan, and if you run out of SPs, remaining will be on-demand. Would that explain what you see in your billing records?
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Thanks @Kallu. Yes, that explain the behaviour in billing records. I dont see any doc where it explain the RI and SP goes hand-in-hand for certain instance family. For Oct 2023 billing, I see 141 instances billed under both RI & SP & majority into m5 and t3 family. I have significant reservation in m5 family through. My worry is that, I am getting around 70% discount on RI where as 50% discount on SP on those 141 instances.