Prometheus alert CPUThrottlingHigh raised but monitoring does not show it
The CPUThrottlingHigh is an alert created by the kubernetes-mixin project. There is an open issue (#108) to discuss this alert. I suggest that you read all the comments on this issue to better understand the problem.
In short, the problem is: When working with low CPU limits, spiky workloads can have low averages and still be being throttled.
Also, take a look at this issue (#67577) from Kubernetes project, which addresses a Kernel bug in CFS quotas that may cause unnecessary CPU throttling. The discussion is still open, and the Kubernetes project are even considering disabling CFS quotas for pods in the Guaranteed QoS (see #70585 for reference).
Consider the following options:

Increase (or even remove) your container CPU limits
Disable Kubernetes CFS quotas entirely
(kubelet's flag --cpu-cfs-quota=false)
Use a Kernel version that contains this fix (torvalds/linux 512ac99)