As discussed in JDK-8210768 this assertion in linkResolver has two problems:

  // For private method invocation we should only find the method in the resolved class.
  // If that is not the case then we have a found a supertype method that we have nestmate
  // access to.
  if (resolved_method->is_private() && resolved_method->method_holder() != resolved_klass) {
    ResourceMark rm(THREAD);
    DEBUG_ONLY(bool is_nestmate = InstanceKlass::cast(link_info.current_klass())->has_nestmate_access_to(InstanceKlass::cast(resolved_klass), THREAD);)
    assert(is_nestmate, "was only expecting nestmates to get here!");

First the nestmate access check should be between the current class and the resolved_method->method_holder() not the resolved class.

Second, we are only assured to have found a nestmate if normal access checking is enabled.

But on further examination of this and a related test case the entire check regarding private methods is incorrect and needs to be removed as it violates the JVMS method resolution rules.