Setting the WiredTiger cache size only limits the memory used for reading and updating documents and indexes in memory (aka the "active working set").
However we see the memory usage go up well into 10GB when we have a lot of traffic. Is that normal behavior?
Yes, more memory usage when you have a lot of traffic should be expected.
Is there another variable that needs to be limited?
The upper limit on resource usage should be determined by your Kubernetes pod or container configuration. Suitable limits to set will depend on your workload, deployment resources, and performance goals.