I've seen several Electric Aid charts by now, and they all seems to have this step at 65% SOC.
This chart shows amperes flowing out of the HV battery while the petrol engine is running and gives an estimate of the electric contribution to the car movement.
It seems that after 65% the HV battery supply a minimum of 10A to the engine, while under 65% lower flows are permitted.
Here's the same trip with a different perspective: engine RPMs are charted only when there's an electric contibution to the petrol engine.
Higher SOC values seem to lead to lower RPMs.