calling task_info in iOS does not return limit_bytes_remaining field.

Considering this code:

var taskInfoData = task_vm_info_data_t()
var count = mach_msg_type_number_t(MemoryLayout<task_vm_info>.size) / 4 // sizeof(natural_t)
let result: kern_return_t = withUnsafeMutablePointer(to: &taskInfoData) {
    $0.withMemoryRebound(to: integer_t.self, capacity: 1) {
        task_info(mach_task_self_, task_flavor_t(TASK_VM_INFO), $0, &count)
    }
}
var physicalFootprintBytes: Int64? = nil
var availableMemoryBytes: Int64? = nil
if result == KERN_SUCCESS {
    // https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/osfmk/kern/task.c#L5396
    // kTaskVMInfoRev1Count is defined in the bridging header
    if count >= kTaskVMInfoRev1Count {
        physicalFootprintBytes = Int64(taskInfoData.phys_footprint)
    }
    // Does not work
    if count >= kTaskVMInfoRev4Count {
        availableMemoryBytes = Int64(taskInfoData.limit_bytes_remaining)
    }
}

I built this code in Xcode 12.5 using the iPhoneOS 14.5 SDK targeting iOS 12.0. I ran this code on an iPhone 11 with iOS 14.8. I expected both the phys_footprint and limit_bytes_remaining fields to have sane values, but limit_bytes_remaining was zero.

TASK_VM_INFO_COUNT in <mach/task.info> is 87 and my code (mach_msg_type_number_t(MemoryLayout<task_vm_info>.size) / 4) is also 87. However, the return value of the count variable is 42. This is equivalent to TASK_VM_INFO_REV2_COUNT in <mach/task.info>.

After reading the XNU code, I found that there is a workaround to handle the wrong length [1]. I think this logic affects this code, but I don't know why.

Is it considered a bug? Or are there any problems with my code? Does the deployment target of the app (currently iOS 12.0) affect this?

[1] https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/osfmk/kern/task.c#L5244