> A credit specification is only available for T2, T3, and T3a instances. Selecting Unlimited for the credit specification allows applications to burst beyond the baseline for as long as needed at any time.
So that would mean Unlimited is not a setting available for T4g (ARM instance) and therefore may explain inconsistent behavior in the ARM instance.
> T4g instances start in unlimited mode by default, giving users the ability to sustain high CPU performance over any desired time frame while keeping cost as low as possible. For most general purpose workloads, T4g instances in unlimited mode provide ample performance without any additional charges. If the average CPU utilization of a T4g instance is lower than the baseline over a 24-hour period, the hourly instance price automatically covers all interim spikes in usage. In the cases when a T4g instance needs to run at higher CPU utilization for a prolonged period, it can do so for a small additional charge of $0.04 per vCPU-hour. In standard mode, a T4g instance can burst until it uses up all of its earned credits. For more details on T4g credits, please see the EC2 documentation page.
I've just launched a test t3 instance and it didn't pop up anything.
They're using KVM under the hood, which supports things like live migration, interrupts to facilitate memory transfer, and so on. At home I'm doing live migration with less than 300ms of interrupts with high memory pressure. It would make sense for them to move things around, just for things like maintenance and distributing load.