Fenix 6 with background app watchface doesn't update

A user has reported my watchface doesn't work with his Fenix 6 but does with his Vivoactive 3. So, he's been helping me debug it a bit and it appears that after the background process (which does a webRequest) calls Background.exit(data), the app's onBackgroundData() method isn't called, until something else, like a smart notification, happens on the watch.

He sees the same behavior with both firmware 9.0 and 10.10 on the Fenix 6. I don't see this problem on the simulator and this is the first report I've received about it.