what has probably happend is that after the 1st fix installation then the second fix the pipework has layed unused for a while (no water supply)and excessive flux has been left to degrade/oxidize. The pipework should have been flushed after the 1st fix and then flushed after the second fix. Excessive boss white use can also cause major problems with valves anf fixtures.
There have been cases with new builds where pipework had been left with debris inside for very long periods and major problems where encountered after water was finally connected needing complete repipes. im unsure without reading the water regs book but in these cases you need to decontaminate the pipework before commissioning of the system.
There have been cases with new builds where pipework had been left with debris inside for very long periods and major problems where encountered after water was finally connected needing complete repipes. im unsure without reading the water regs book but in these cases you need to decontaminate the pipework before commissioning of the system.