Similar issue, I purchased a usb power meter that sits in line with usb device, it shows full output sometimes but not always. I plugged my phone into it and rode 6 hours, no headlights just led runnig lights, the phone was almost flat at the end. I saw somewhere that the usb power is regulated by the ecu and shuts it down at times of high alternator load. Not convinced it works properly at all.
Now installed a seperate feed to a usb charger module froma switched supply, works great.