Thanks for the follow-up. Liked the OpenDoc reference.
I think I've finally figured out why this was all happening. The editing features in the Photos application (both for iOS and macOS/OSX) are slightly different between versions. In the iOS 10 and macOS versions of the Photos application, another editing option under the "Light" section for the Adjust tool was added, that option is "Brilliance". So in both of those versions (iOS 10.3, macOS 10.12), here are the adjustments that can be made under the "Light" section:
Brilliance
Exposure
Highlights
Shadows
Brightness
Contrast
Black Point
For iOS 9 and OS X 10.11 (El Capitan), the Photos application does not have the Brilliance adjustment, just the six listed from Exposure down.
Had to borrow an older phone and newer Mac to finally figure this out. But it looks like for my own setup, when I edit in Photos.app on iOS 10.3.3, it's making adjustments that Photos.app on my MacBook running El Capitan 10.11.6 doesn't understand due to the extra adjustment in the newer version of iOS, so it just throws out all the adjustments.
Basically, edits in Photos from iOS 9 will be maintained when imported into Photos on OS X 10.11 (El Capitan). And edits in iOS 10 will be maintained when imported into Photos on macOS 10.12 (Sierra). Edits were not preserved otherwise.
(Note, I don't know if there are versions in between that work, I was only able to test with the latest versions of iOS9, iOS10, OS X El Capitan, and macOS Sierra)