com.apple.driver.usb.cdc.acm driver fails to load on live insertion of matching usb device (e.g., an Arduino)

Hi, folks, have others noticed this problem?

Fairly recently during a Sierra or High Sierra update, a suitable USB kext (driver) failed to load when an Arduino Uno-clone was connected to a running MacBook Pro. The USB device would appear in the System Report, but no serial device (for the Arduino) would appear and be discoverable by the Arduino IDE (or other apps using a serial port, e.g., CoolTerm). Disconnecting and reconnecting would sometimes work, but lately ceased to work at all. Rebooting the Mac, with the Arduino already connected did work, and the device would show up with an active serial port.

After chasing down several blind alleys (e.g., incompatibility between FTDI and CHx USB chips), the following appears to be the true cause: the AppleUSBACM (Version 5.0.0) Driver, with ID com.apple.driver.usb.cdc.acm loads on a MacOS reboot with the Arduino Uno already connected and presents a serial port device at path /dev/cu.usbmodem1A21, and works OK. But if the USB device is disconnected and reconnected (or connected the first time) in a running macOS, the kext fails to load and the serial device never appears.

Workarounds:

1. Reboot with the device connected.
2. Run the Serial app, and then the "File | Open Port…" command, which will open a window with a list of the currently active serial ports. This list is initially blank, but after a few seconds, the Arduino device will appear with the cu.usbmodem1a21 name. That is, the Serial app has done something to force the AppleUSBACM driver to load and make the serial device active. (Note: Serial is even smart enough to only list active serial ports, not including others, e.g., Mike's-iPhone6S, that were previously installed but are currently not available.)