The exact same thing happened to me: package went from Shenzhen to Hong Kong; left Hong Kong, lands in Alaska, then departs and arrives in Kentucky. Then 3 days later, it shows up in Hong Kong again.
I got some weird reason for the issue as well. At first in an online chat with an Apple tech, he told me the UPS site was experiencing problems, despite the fact my other UPS items were tracking fine. When I brought up this discrepancy, he basically just reiterated it was a glitch and kind of left it at that.
When 24 hours passed, and it still wasn't updating, I called Applecare this time, and someone in sales told me that the iPad's had not passed through customs, and apparently are just sitting on a pallet somewhere waiting to be shipped. She told me she'd give me free expedited shipping as a consolation, but there was still no definitive date given. She stated that tracking should update 24 hours from local Hong Kong time, but it's been almost 48 hours Hong Kong time since this all happened, and there's been no change in my tracking info.
I have no idea what's going on; I don't imagine customs would take this long. I checked the UPS site for info on international packages and their customs clearance time, and it stated quote: "virtually all [packages] are cleared immediately".
Honestly, while I appreciate the professional candor of Apple tech support, I agree that this information seems bogus. First it was a glitch, now it's a customs issue; I'm pretty much forced to conclude that this is either some once-in-a-blue-moon shipping error, or more realistically, an issue on Apple's end.
The iPad Pro I ordered was Space Gray, 128GB, with Wifi only, if that helps.