I asked miles here in the UK about this and this is his reply
The slight "stall" is the unusual effect of having an electric motor working on a 2 speed gearbox. It uses the gears on the EV side to effectively give you 10 gears overall (with the Petrol engine and motor working together). When in hybrid mode, the engine power is able to mask the motor's gear change, but in EV mode, there is nowhere to hide!
In Hybrid mode, the car uses the logic that the EV is most suited to lower speeds (sub 25mph urban traffic), and the engine is more suited to faster road speeds. So on a mixed commute, it will try to use the engine above 25mph to eke out the EV range further (for example, if you do a commute which involves leaving an urban area, driving between towns, and then driving back into an urban area).
The ZS EV only has one gear, so doesn't have this effect. It's only the PHEV which has the two-speed gearbox on the EV motor.