AVAudioEngine input tap intermittently delivers all-zero buffers — valid format, no error thrown

We have a long-form audio recording app built on AVAudioEngine. We install a tap on inputNode, accumulate the PCM buffers, and encode them to AAC in ~60-second chunks. Setup is essentially:

let session = AVAudioSession.sharedInstance()
try session.setCategory(.playAndRecord, mode: .default,
                        options: [.defaultToSpeaker, .allowBluetooth, .allowBluetoothA2DP])
try session.setActive(true)

let engine = AVAudioEngine()
let input  = engine.inputNode
let format = input.inputFormat(forBus: 0)   // valid, e.g. 48 kHz, 1 ch

input.installTap(onBus: 0, bufferSize: 1024, format: format) { buffer, _ in
    // In the failure case, buffer.floatChannelData is entirely 0.0
    // (accumulate + encode to AAC)
}
engine.prepare()
try engine.start()

Intermittently — and so far only reported from the field, never reproduced in normal testing — a recording comes out completely silent. When we decode the resulting AAC and inspect the raw PCM, every sample is exactly 0.0. The signature is very specific:

• The engine is running and the tap keeps firing for the full duration (normal number of buffers / full-length chunks).
• inputFormat is valid (sampleRate ≠ 0, e.g. 48 kHz).
• No error is thrown anywhere — setCategory, setActive, start(), and the tap callback all succeed.
• The PCM is literally all zeros (not low-level noise / room tone — exact 0.0).
• Two separate silent recordings decode to byte-identical AAC, confirming pure digital silence rather than corruption.

So as far as our error handling, format checks, and tap-liveness are concerned, everything looks healthy — yet the microphone is delivering pure silence.

One way we can reproduce it: recording while the iPhone is being driven via macOS iPhone Mirroring (the iPhone stays locked, the mic is effectively unavailable from the device, but our session still activates with a valid format and the tap fires zero-filled buffers for the whole recording — with no error at any point).

What we've ruled out: microphone permission is granted; it's not truncation or short capture (full-length, full frame count); it's not our encoding step (the input buffers themselves are zero); it's not a quiet/obstructed mic (that would be low noise, not exact 0.0).

Questions:

1. What other device states or scenarios can cause a running AVAudioEngine input tap to deliver all-zero buffers with a valid format and no error? (e.g. another process/system feature holding the mic, Continuity Camera/Mic, CallKit/PushToTalk session ownership, etc.)
2. Since this surfaces with no error and a valid format, what is the recommended way to detect it at runtime? Is monitoring the input level / PCM energy the only signal, or is there a supported API to know the input isn't actually live?
3. What's the recommended recovery once detected — is a full session deactivate/reactivate re-handshake sufficient, or is recreating the engine required?