My impression is that homes in the US tend to have more wood involved in their construction generally, so it seems plausible to me that US construction methods may be more experienced with ways of managing wood moisture

Sounds like the foam is the issue, which may be down to the shade tree handy man they hired being unlicensed or uninsured to perform insulating. If a person isn't a dedicated HVAC worker who has day-to-day experience with insulation types and how to properly apply them, then the work may be causing more harm than good.

If the insurance and mortgage companies have a list of guidelines needed for insulation installation, it makes sense that folks need to follow them to continue using their services. Spraying any old foam into nooks and crannies is a fine way of hiding problems.

The problem with this style of foam application, is that you cannot get to the wood to check the moisture content, without ripping down the foam (or probing through it, exacerbating any problem).
So there isn't a way to test. And as there have been problems, the lenders won't take the risk on any more.