If you experience this effect, it is due to the installed USB-C Power Delivery module.
If you have already plugged in a device (e.g. tablet) and plug in another device that you want to supply with power (e.g. cell phone), the USB module redistributes the load and temporarily interrupts the power supply.
RECOMMENDATION: Please always use the included power supply for your devices that are to be permanently supplied with power (e.g. Fire TV Stick, Google Chrome Cast, etc.)!