The answers from others already indicate that the fault lies somewhere in the system where normal visual inspection is not sufficient. In my opinion, the question about possible dead pipes is actually crucial.
Was any branch from the wastewater for a sink/floor drain in the utility room installed but never connected? It was something similar for us in the HAR behind a boxed-in area.
Even a single open sleeve, which was forgotten or only poorly sealed, acts like a permanent entry point for odors – especially in combination with ventilation or negative pressure situations, as you repeatedly describe with the KWL.
What I would be interested in: Have you already had someone come who specifically inspected the vent pipe and wastewater routing for leaks using a fog machine or smoke cartridge? That often closes all open questions faster than mere guessing in the fog.
And when you write that the musty smell can be traced upward along the entire pipe run, was the tightness around the stack really checked the last time it was opened, or did the fitter only fix a visible defect?