Do you know how big your emitter circuit is? The heatpump app reckons it’s 12 kW at dT 50. At a flow temperature of 35° the radiators can only emit 2kW, whereas the heatpump has a minimum output of 3.7 kW.
At 40° flow, 3 kW output from the emitters are more able to keep up, though is still less than it needs so some cycling is inevitable.
I reckon the target temperature for WC is simply too low for your emitter circuit, and the heat pump will not run smoothly below 43°C. Setting a higher curve may give you better performance.
See Calculating the optimal weather compensation curve for my spreadsheet.