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The issue is that it is not piped properly and the installer will have to correct it.
The fact that it drains when you run the sink is the proof that it is not piped correctly. It is not properly vented (I am in the U. S. so the terminology may not be the same ) and the tub is getting air locked. When you run the water in the sink it breaks the air lock. They would have to add an air admittance valve to the sink.