As @alzhao wrote before, the poweroff command all OpenWRT routers does not power off the router as it would in a PC or a phone.
What basically happens when you run the poweroff command is, all open file handles are closed, programs are all stopped and the router sits in idle. Think of “Safely Remove Hardware” in Windows. You would run the poweroff command before unplugging the router to prevent file corruption in “mission critical” applications, or when you need to constantly power off the router and there is a chance a file is being written to, or is in use.
Unfortunately this means that the network stack, power regulation and everything else is running as normal and drawing power.
If you want to do a full power down, you would need to add a relay or a latching power switch like this Pololu - Mini Pushbutton Power Switch with Reverse Voltage Protection, LV and control it via a GPIO pin.