AUAudioUnit VoiceProcessingIO not calling inputHandler on macOS

Question

I'm trying to use the new AUAudioUnit API in Swift to use the VoiceProcessingIO unit on both iOS and macOS like so:

var description = AudioComponentDescription()
description.componentType = kAudioUnitType_Output
description.componentSubType = kAudioUnitSubType_VoiceProcessingIO
description.componentManufacturer = kAudioUnitManufacturer_Apple
description.componentFlags = 0
description.componentFlagsMask = 0

var vpio: AUAudioUnit
vpio = try! AUAudioUnit(componentDescription: description)

print("canPerformInput:", vpio.canPerformInput)
print("isInputEnabled:", vpio.isInputEnabled)
print("canPerformOutput:", vpio.canPerformOutput)
print("isOutputEnabled:", vpio.isOutputEnabled)
print("---")

vpio.isInputEnabled = true

vpio.inputHandler = { (actionFlags, timestamp, frameCount, inputBusNumber) in
    print("inputHandler called")
}

vpio.outputProvider = { (actionFlags, timestamp, frameCount, inputBusNumber, inputData) -> AUAudioUnitStatus in
    print("outputProvider called")
    return noErr
}

try! vpio.allocateRenderResources()
try! vpio.startHardware()

This code works on iOS (10.2) , it calls both the inputHandler and the outputProvider:

canPerformInput: true
isInputEnabled: false
canPerformOutput: true
isOutputEnabled: true
---
inputHandler called
outputProvider called
inputHandler called
outputProvider called
...

On macOS (10.12.2) however, only the outputProvider gets called:

canPerformInput: true
isInputEnabled: true
canPerformOutput: true
isOutputEnabled: true
---
outputProvider called
outputProvider called
outputProvider called
outputProvider called
...

Observation: On iOS, isInputEnabled is set to false by default, and has to be enabled (line 17) for the inputHandler to be called. On macOS, it is enabled by default, but the inputHandler isn't called. Explicitly setting isInputEnabled like above doesn't work either.

VPIO works fine on macOS when using the old API (AudioUnit, AudioComponentInstanceNew, AudioUnitSetProperty, kAudioOutputUnitProperty_SetInputCallback etc...) as used in Apple's echoTouch sample code and suggested here by theanalogkid (Except that on macOS you cannot and don't need to set kAudioOutputUnitProperty_EnableIO)

Am I doing something wrong here or could this be a bug on Apple's part?