If you use poweroff or halt command, the Linux actually halt, meaning it is not rebooting, not running. But does not power off the chipset.
If you have Firmware v4.x, pls try the following command to turn off. This should work for both E750V1 and E750V2
ubus call mcu cmd_json {\"poweroff\":\"1\"}
About Auto Power on when plugging power, E750V2 will NOT auto power on when plugging power.