iOS 11.4.1 may require users to unlock their passcode-protected iOS devices in order to connect them to a PC, Mac or a USB accessory after one hour since the device has been last unlocked or disconnected from a trusted USB accessory. An iPhone in USB Restricted Mode acts as a dumb battery pack: in can be charged, but cannot be identified as a smart device. This effectively blocks forensic tools from being able to crack passcodes if the iPhone spent more than one hour locked.”]
Source: https://blog.elcomsoft.com/2018/07/this-9-device-can-defeat-ios-usb-restricted-mode/