Hello @ADCish 

Sorry for the late reply, can you help describe how the LED change when powerline adapter turn off? 

Do you mean on one of the PA4010 units, all the led will start blink then go off, and will light up again?

• Try a factory reset the unit and repair 2 devices

- Reset Wireless powerline unit:  Press and hold the Reset button (or use a pin to press reset hole) until all LEDs turn off and start on again.          
- Reset Wired Powerline unit: Press the pair button for more than 5 secs until most Led flash once           

• Try a different wall outlet make sure not bad contact  (skip this if you've tried this)
• Update device firmware 

Firmware for PA4015（FR）: #Firmwarehttps://www.tp-link.com/fr/support/download/tl-pa4015p/#Firmware

Note: (select Hardware according to the back labelof device before download corresponding firmware )

If still the same concern persists, may I know how long do you have the unit, if device has been working for a long time, it is recommended to contact retailer or local support to check device warranty.