So that was my first thought that the primaries were pipe within pipe and the outer had holes for the flow and return to the heatpump, but being that close together you would have very little stratification or none and potentially your heat pump is going round in circles without the full flow rate going to your plate to plate.
Not to everyone’s taste but I would ditch the glycol use frost protection valves or the defrost setting. Glycol is not good for heatpumps after so long it becomes syrup like and can cause more issues.
Just looked at the instructions and the bottom connections are the ones that are double piped but this is still a poor design for stratification. With a back up heater fitted the the bottom connections would be for the flow and return to the circuit instead of being the primaries as they are now.