Google Cast SDK - iOS - Why can't I connect to a custom receiver?

Question

My ap works fine with the default cast receiver app, but will not connect to my custom receiver app.

The main thing I would like to find out is what is this log message:

GCKCastDeviceMDNSScanner MDNSServiceBrowser:didUpdateService

I cant find anything on it by googling.

---

So, the setup:

Using GoogleCast 4.6.1 xCode 14.3.1

Using the default receiver, I can send media to play:

1. connect to device
2. open receiver app
3. connect to app
4. play media stream

When I change to use a custom receiver (switching the applicationID in GCKDiscoveryCriteria), it fails on 3-connect to app. It opens the app on the device, but then just freezes, eventually timing out.

I get this log msg instead of the GCKCastDeviceController connectToApplication I am expecting:

GCKCastDeviceMDNSScanner.m:457: -[GCKCastDeviceMDNSScanner MDNSServiceBrowser:didUpdateService:] - updated service 'Chromecast-*{device_id}*'; subtypes: {(
    "_*{application_id}*"
)}

When searching for "MDNSServiceBrowser" "didUpdateService" on Google, I get nothing?!