Alexa commissioning stops after `attestationRequest` and expires the failsafe

Describe the bug Alexa cannot commission Matterbridge. The Alexa app first reports that Matterbridge cannot be detected and asks me to confirm that the device is in pairing mode. At the end of the flow it displays: Something went wrong. Error: GS014 in japanese.

Matterbridge is actually discovered successfully, but Alexa stops sending commissioning commands immediately after Matterbridge responds to attestationRequest. The failsafe expires after the 80 seconds.

I initially had Matterbridge commissioned to Google Home. I removed the Google Home fabric and retried with no registered fabrics, so Alexa was the first and only commissioner. The result was unchanged.

To Reproduce

1. Start Matterbridge with matterbridge --mdnsinterface eth0.
2. Ensure that no existing fabrics are registered.
3. Open a new commissioning window from the Matterbridge frontend.
4. Scan the newly generated QR code with the Alexa app.
5. Wait for commissioning to complete.

Expected behavior Alexa pairing, and adds its fabric to Matterbridge.

Debug file Matterbridge was run in the foreground. These logs were captured from its standard output; I could not find a corresponding log file on disk.

[15:28:16.115] [MdnsAdvertisement] Publishing kind: commissionable service: mdns:F49A7FBBDF50FCE1._matterc._udp.local
[15:28:25.395] [PaseServer] Received pairing request « udp://<ECHO_SHOW_LAN_IP>:5541
[15:28:25.471] [PaseServer] @0:0•320b New session with udp://<ECHO_SHOW_LAN_IP>:5541 5↔2
[15:28:25.476] [Session] •unsecured#38f7ec425bb4686f Session ended
[15:28:25.487] [InteractionServer] Invoke « @0:0•320b⇵d755 invokes: 0.generalCommissioning.armFailSafe
[15:28:25.488] [ProtocolService] Invoke « Matterbridge.generalCommissioning.armFailSafe @0:0•320b⇵d755✉05876964 expiryLengthSeconds: 80 breadcrumb: 1
[15:28:25.489] [ProtocolService] Invoke » Matterbridge.generalCommissioning.armFailSafe @0:0•320b⇵d755✉05876964 errorCode: 0 debugText:
[15:28:25.606] [InteractionServer] Invoke « @0:0•320b⇵d75a invokes: 0.generalCommissioning.setRegulatoryConfig
[15:28:25.607] [ProtocolService] Invoke « Matterbridge.generalCommissioning.setRegulatoryConfig @0:0•320b⇵d75a✉0587696e newRegulatoryConfig: 2 countryCode: JP breadcrumb: 0
[15:28:25.608] [ProtocolService] Invoke » Matterbridge.generalCommissioning.setRegulatoryConfig @0:0•320b⇵d75a✉0587696e errorCode: 0 debugText:
[15:28:25.635] [InteractionServer] Invoke « @0:0•320b⇵d75c invokes: 0.operationalCredentials.certificateChainRequest
[15:28:25.635] [ProtocolService] Invoke « Matterbridge.operationalCredentials.certificateChainRequest @0:0•320b⇵d75c✉05876972 certificateType: 1
[15:28:25.636] [ProtocolService] Invoke » Matterbridge.operationalCredentials.certificateChainRequest @0:0•320b⇵d75c✉05876972 certificate: (snip)
[15:28:25.641] [InteractionServer] Invoke « @0:0•320b⇵d75d invokes: 0.operationalCredentials.certificateChainRequest
[15:28:25.641] [ProtocolService] Invoke « Matterbridge.operationalCredentials.certificateChainRequest @0:0•320b⇵d75d✉05876974 certificateType: 2
[15:28:25.642] [ProtocolService] Invoke » Matterbridge.operationalCredentials.certificateChainRequest @0:0•320b⇵d75d✉05876974 certificate: (snip)
[15:28:25.648] [InteractionServer] Invoke « @0:0•320b⇵d75e invokes: 0.operationalCredentials.attestationRequest
[15:28:25.649] [ProtocolService] Invoke « Matterbridge.operationalCredentials.attestationRequest @0:0•320b⇵d75e✉05876976 attestationNonce: (snip)
[15:28:25.650] [ProtocolService] Invoke » Matterbridge.operationalCredentials.attestationRequest @0:0•320b⇵d75e✉05876976 attestationElements: (snip)
[15:29:45.489] [FailsafeContext] Failsafe timer expired; resetting fabric builder
[15:29:45.490] [Session] @0:0•320b Session ended
[15:29:45.491] [MdnsAdvertisement] Publishing kind: commissionable service: mdns:BEBDBAF00F581CE2._matterc._udp.local
[15:29:45.624] [ExchangeManager] @?:?•320b Ignoring message for unknown session
[15:29:46.236] [ExchangeManager] @?:?•320b Ignoring message for unknown session
[15:29:46.792] [ExchangeManager] @?:?•320b Ignoring message for unknown session
[15:29:47.824] [ExchangeManager] @?:?•320b Ignoring message for unknown session
[15:29:49.334] [ExchangeManager] @?:?•320b Ignoring message for unknown session

Plugin:

• matterbridge-webhooks: 1.3.0

Matterbridge:

• Matterbridge: 3.10.5
• Node.js: v24.18.1
• OS: Arch Linux, kernel 7.1.8-arch1-3 on proxmox VM
• Firewall: disabled
• Start command: matterbridge --mdnsinterface eth0
• Runtime user: unprivileged local user
• Network interface: eth0
• Interface MTU: 1500
• IPv6: enabled, with both global and link-local addresses configured
• Regulatory country code sent by Alexa: JP
• Alexa app version: 2.2.698932.0
• Alexa app device: Google Pixel 6a running Android 16
• Alexa Matter controllers:
  • Echo Dot with Clock, device software version 13121734532
  • Echo Show 5 (3rd generation), device software version 5601010007320
• Matterbridge and the Alexa commissioner are on the same LAN

Additional context

• The issue reproduces with both listed Alexa Matter controllers (Echo Show, Echo Dot with Clock).
• The same Matterbridge instance commissioned successfully to Google Home, and its bridged device could be controlled normally.
• Removing the existing Google Home fabric did not change the behavior.
• The final error displayed by the Alexa app is GS014.