That sounds exactly what mine does.The 400 series ecotecs light at around 2/3 of the appliance output regardless of the D0 setting. This is to establish a stable flame on the burner before it can ramp down. If the boiler is oversized for the load it can overshoot the demand temperature while still in the ignition phase.
I was really hoping that it would be different to this when being controlled using the ebus (not the 230 V input), by having a Vaillant thermostat / controller / wiring centre. I guess this is especially useful, when the weather is mild, and you only need a low flow temperature, as in this situation, during the first minute, the flow temperature could well rise too much. If it does rise too much, would the boiler cut off, or would it cope with that, and modulate down still?