I’ve had similar flow rate reporting with my Sharky during hot water runs.
The Sharky goes the other way and sends out crazy high numbers when it errors.
I think it’s due to placement and turbulence caused by the hot water run on a shorter circuit than the heating circuit.
You have to adhere to the instructions for placement, ie not near bends. See the Sharky thread for examples.
Can you take a photo of how your meter is placed/installed?
Does the Kamstrup show any errors during the DHW run? What flow does it report on the screen during this time?