Just let the heating engineer come now and fix it.
If it were that easy, the heating engineer was commissioned by the general contractor and is 400km away. Before anything happens, I have to first submit an official defect notice with a deadline.
That's where you can't avoid if nothing actually gets through. In the bathroom, your heating surfaces are probably already barely sufficient, so you definitely need flow on the third circuit.
I am just wondering how big the bathroom must be to have 3 heating circuits installed here? I would now also contact the heating engineer, who has completely different options here ([Durchspülen] etc.)
Actually, it doesn't really matter, but I would also like to know why 3 circles were laid there for an estimated area under 10 m². The middle one covers barely 2 m²...