Well, looks to be not just an Adobe problem. I just looked in Apple's Image Capture and the JPGs are rendering the same incomplete way.
Would love to have your opinion though that even though it is not an Adobe issue at this moment, where do you think the problem might lie?
Thanks for the confirmation. This looks like a macOS bug then given that Apple Image Capture is exhibiting the same issue. Lightroom is relying on the same macOS Image Capture API to communicate with the devices (iPhone in your case), thus it is not surprising that both Lr and Apple Image Capture exhibits the same symptons.
I'll relay the info to one of our devs to do some investigation and maybe report the issue to Apple. In the meantime, I don't have a good suggestion for a workaround. Maybe update to the latest iOS and/or macOS would help. And call ProCamera tech support to see if they are aware of such issues.