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@absurdbird
I reckon its a uphill section of pipe.
When you have a large enough body of water (filling sink with plug in) it has enough momentum to go over the negative drainage section then siphons the rest of the water along.
It may be an air admittance issue but usually the symptom that requires that remedy is a siphoning pipe -it draws the water from the trap into waste pipe.
I'm with Dammit get the fitter/plumber you paid to fit it to sort it out.