I'll close the loop on this - it just took time. The magic threshold of 5 unique devices must be more complicated than just that. Eventually, the reports did come through. So if you use campaigns to generate a link - just wait more time.

Apple developer enterprise program is not used to distribute apps via the normal App Store. Its more for internal app distribution within your own company.

I get that this is 6 years old - but did you happen to find the answer out?