We have the same issues on multiple assets after switching to 5.6
The only temporary workaround is to do
if(Object->HasAnyFlags(RF_NeedLoad) && Object->HasAnyFlags(RF_WasLoaded))
{
Object->ClearFlags(RF_NeedLoad);
}
before the failed check code ( We are also hitting the same check in UObject::ConditionalPostLoad() )
It could potentially lead to other issues, but at least you could open the editor until the proper fix is made