I finally got his working on the latest Xcode v15.4 and WatchOS v10.5 releases! I had to restart my MacBook after updating everything.

Did you ever solve this problem?

Whoops, never mind. It was just a bug related to the initialisation of DeviceActivityEvent in my code.

I've also got this issue. I'm pretty sure it's also causing the screen to hang for a while while it loads the labels.