Passive Roaming Error with Packet Broker - "channel closed" on GKE with Ingress

Hi everyone,
I'm running ChirpStack in Google Kubernetes Engine (GKE) and using an Ingress for external access. However, when trying to connect to Packet Broker, I'm encountering an error.

Setup:

# Backend Interfaces configuration (optional).
[backend_interfaces]
bind="0.0.0.0:8181"
ca_cert="/etc/chirpstack/certs/ca.pem"


[roaming]
  #resolve_netid_domain_suffix=".netids.lora-alliance.org"
  

  [[roaming.servers]]
  
    net_id="000013"
    enabled=true
    
    async=true
    async_timeout="30s"
    
    passive_roaming=true
    passive_roaming_lifetime="0s"
    
    server="https://eu.packetbroker.io:5138"
    

    authorization_header="Key xxx.XXX"

Error:
Making request server=https://eu.packetbroker.io:5138 async_interface=true
backend: Async response received
Start passive-roaming error net_id=000013 error=channel closed

The request to eu.packetbroker.io:5138 seems to go through and an async response is received, but then passive roaming fails with "channel closed".

My suspicion:
Does port 8181 need to be exposed directly (e.g., via LoadBalancer instead of Ingress)?
Has anyone successfully set up ChirpStack + Packet Broker on GKE or in general?

Any guidance on configuration or best practices would be greatly appreciated!
Thanks in advance!