Q: Why does the right-hand USB 3 port on my MacBook Pro (mid-2015) periodically lose USB 3 connectivity?

The circumstances that cause this are obscure, but here are the facts when the port isn't working as it should:

1. Neither USB 3 external hard drive (2 different enclosures and 2 different cables) from OWC using the ASMedia 1053e chipset) will mount on right USB port (will however mount when connected to left USB port)
2. When this happens, "System Information" app shows no new device on USB bus
3. However, I can connect an iPhone 6S to the right port and it will show up in “System Information”
4. If I create a new user account and connect the external hard drive to the right port, no change (won’t mount)
5. Restarting and/or resetting SMC fixes the problem

This problem occurred under El Capitan and currently occurs under Sierra.

Bug or hardware problem?