The book value of the property is related to prospective rental income. It is - bizarrely - sometimes more profitable for owners and investors to maintain the fiction of high rental value without any income than to drop the rental value to something realistic and take a realised loss. Even if that's generating real income.

I would guess it's the same in the US.

This leans suspiciously towards subprime-all-over-again, where the nominal value and security of investments is being wildly overstated.

At some point it's going to have be unwound, which will create some interesting readjustments.