There appears to be a bug in the Generic Zigbee Outlet driver that is causing the issue. I was able to get power reporting to work on my Sinopé Outlet by switching to Steve's Enhanced Smart Plug Driver and then switching back to the Generic Zigbee Outlet.
Steps I took:
- Changed driver to Iris Smart Plug (this is the Enhanced Smart Plug driver)
- Clicked 'Save Device'
- Clicked Configure
- Turned on the Outlet
- Saw power correctly shown, but voltage and AC frequency were way off
- Turned off the Outlet
- Changed driver to Generic Zigbee Outlet
- Clicked 'Save Device'
- Clicked 'Configure'
- Turned on the Outlet
- Saw power still being shown correctly
- Drank a beer to celebrate (optional)
Edit: After updating to version 2.2.5, you might only need to click 'Configure' on the device page.