What is the right behaviour now?

Your choice to accelerate is correct because, at a construction site with a reduced speed limit, you must adjust your speed accordingly. When you pass the obstacle and see the speed limit sign removed or the end of the construction zone indicated, it is appropriate to accelerate back to the normal speed for that road, provided it is safe to do so.

German traffic rules require drivers to reduce speed to the posted limit near hazards like construction sites and maintain that speed until the hazard ends. Once the temporary speed restriction is lifted, accelerating is allowed to resume normal traffic flow. Maintaining a lower speed beyond this point without reason can cause unnecessary delays, while accelerating too early or inappropriately can be dangerous.

In summary, you should slow down when entering a construction zone, obey the reduced speed limit, and then accelerate after passing the hazard only when it is safe and the speed restriction is officially lifted. This is why accelerating now is the right behavior.