A friend of mine is microbiologist who worked for two councils as an environmental health officer. At times she was called in to inspect rental properties that tenants claimed were not fit to be lived in due to mould problems. In almost every case, the mould problem was not caused by the building, but the lifestyle of the occupants. The windows would all be shut tight “to keep the cold out”, and/or there would be laundry drying in front of the heater. Some would never use the exhaust fan in the bathroom… the list goes on.
If there’s a mould problem in a new building, then the only other possibility would be a long term leak. It sounds like there may have been a once-off event that allowed water ingress, but this shouldn’t cause a mould problem.