*Update*
I received the new Garmin 520 Plus and had a chance to test it today on a ride.
The new 520+ was paired with my iPhone 7 per instructions, I had set the Garmin Connect app settings to use Cellular data, and Background app Refresh.
Once I finished my ride, I saved it on the 520+ and observed that the ride uploaded to Garmin Connect (and Strava) without having the Garmin Connect app on my iPhone open.
This is contrary to what the Garmin tech told me. He said that I had to open the Garmin Connect app in order for the uploads to occur. I also confirmed this with several of my riding mates, and they do NOT have to open the Garmin Connect app for the upload to work.

So the good news is that the new 520+ works correctly. The bad news is that Garmin Support Tech didn't know how their products are supposed to work.
I'm going on another ride tomorrow and will be curious to see if this continues to work as expected.

RayK