There’s no galvanic isolation on the CAN bus. According to the ISO 11898-2 the CAN_H and CAN_L should be referenced to the common ground and the common-mode voltage on the CAN pins should not exceed 32V for the 24V power supply. The CAN transceiver IC on the Myxa can withstand up to 58V common mode voltage and 27V differential voltage on these pins. You have to connect the ground pins of all nodes to keep the common-mode voltages within limits. However, the CAN GND is galvanic connected to the Battery “-” too, either on the flight controller side or on the ESC node side. It will work with only the CAN_H and CAN_L pins connected, therefore. Thus, the ground wires are not required in this case, but they are recommended to be used to prevent the common-voltages from going outside the specified limits.