When compressor is not running, the dT between return and flow is zero because no heat is being produced by the heat pump. The app does not show radiator output during this phase as there’s no way to know the dT across the radiators.
The optimal state to calculate emitter spec is during the steady state when power and temperatures are level. The orange line shows the radiator capacity at that mean flow temperature, and so long as the generated heat is below that line, then the heat pump will settle into a stable state. If the radiators cannot emit that heat being generated, then the flow temperature will rise and the heat pump will cycle. This is correct and as expected.
The calculation for system volume is trickier, and works best when the emitter spec is known and the system is heating up from cold. There are some manual steps you can do to get a more accurate reading. For more information about this, see Infer radiator spec and system volume using the MyHeatpump app