I keep hitting error 429 "Resource Exhausted" and my quota seems to be ignored

Although I have a RPM of 10k in Tier two, I get throttled at around 150, is it a bug in detecting the API Tier? And if so what is the point in moving to Tier 2

Ps. it is not clear to me if the issue is concurrency as I tend to run 5 calls in parallel, but I am WAY below the 10k quota.